Adjust documentation for devel deprecation.

Update the documentation to be consistent with #7688.

Library/Homebrew/formula.rb

@@ -2383,6 +2383,7 @@ class Formula
     #   depends_on "cairo"
     #   depends_on "pixman"
     # end</pre>
+    # @private
     def devel(&block)
       @devel ||= SoftwareSpec.new
       return @devel unless block_given?
@@ -2415,7 +2416,7 @@ class Formula
     end
 
     # Additional downloads can be defined as resources and accessed in the
-    # install method. Resources can also be defined inside a {.stable}, {.devel} or
+    # install method. Resources can also be defined inside a {.stable} or
     # {.head} block. This mechanism replaces ad-hoc "subformula" classes.
     # <pre>resource "additional_files" do
     #   url "https://example.com/additional-stuff.tar.gz"
@@ -2525,7 +2526,7 @@ class Formula
     #   sha256 "c6bc3f48ce8e797854c4b865f6a8ff969867bbcaebd648ae6fd825683e59fef2"
     # end</pre>
     #
-    # Patches can be declared in stable, devel, and head blocks. This form is
+    # Patches can be declared in stable and head blocks. This form is
     # preferred over using conditionals.
     # <pre>stable do
     #   patch do

docs/Formula-Cookbook.md

@@ -476,7 +476,7 @@ patch :p0 do
 end
 ```
 
-[`patch`](https://rubydoc.brew.sh/Formula#patch-class_method)es can be declared in [`stable`](https://rubydoc.brew.sh/Formula#stable-class_method), [`devel`](https://rubydoc.brew.sh/Formula#devel-class_method), and [`head`](https://rubydoc.brew.sh/Formula#head-class_method) blocks. Always use a block instead of a conditional, i.e. `stable do ... end` instead of `if build.stable? then ... end`.
+[`patch`](https://rubydoc.brew.sh/Formula#patch-class_method)es can be declared in [`stable`](https://rubydoc.brew.sh/Formula#stable-class_method) and [`head`](https://rubydoc.brew.sh/Formula#head-class_method) blocks. Always use a block instead of a conditional, i.e. `stable do ... end` instead of `if build.stable? then ... end`.
 
 ```ruby
 stable do
@@ -532,9 +532,9 @@ Instead of `git diff | pbcopy`, for some editors `git diff >> path/to/your/formu
 
 If anything isn’t clear, you can usually figure it out by `grep`ping the `$(brew --repo homebrew/core)` directory. Please submit a pull request to amend this document if you think it will help!
 
-### Unstable versions (`devel`, `head`)
+### Unstable versions (`head`)
 
-Formulae can specify alternate downloads for the upstream project’s [`head`](https://rubydoc.brew.sh/Formula#head-class_method) (`master`/`trunk`) or [`devel`](https://rubydoc.brew.sh/Formula#devel-class_method) release (unstable but not `master`/`trunk`).
+Formulae can specify an alternate download for the upstream project’s [`head`](https://rubydoc.brew.sh/Formula#head-class_method) (`master`/`trunk`).
 
 #### `head`
 
@@ -559,21 +559,6 @@ class Foo < Formula
 end
 ```
 
-#### `devel`
-
-The [`devel`](https://rubydoc.brew.sh/Formula#devel-class_method) spec (activated by passing `--devel`) is used for a project’s unstable releases. `devel` specs are not allowed in Homebrew/homebrew-core.
-
-A `devel` spec is specified in a block:
-
-```ruby
-devel do
-  url "https://foo.com/foo-0.1.tar.gz"
-  sha256 "85cc828a96735bdafcf29eb6291ca91bac846579bcef7308536e0c875d6c81d7"
-end
-```
-
-You can test if the [`devel`](https://rubydoc.brew.sh/Formula#devel-class_method) spec is in use with `build.devel?`.
-
 ### Compiler selection
 
 Sometimes a package fails to build when using a certain compiler. Since recent [Xcode versions](Xcode.md) no longer include a GCC compiler we cannot simply force the use of GCC. Instead, the correct way to declare this is the [`fails_with`](https://rubydoc.brew.sh/Formula#fails_with-class_method) DSL method. A properly constructed [`fails_with`](https://rubydoc.brew.sh/Formula#fails_with-class_method) block documents the latest compiler build version known to cause compilation to fail, and the cause of the failure. For example:
@@ -763,7 +748,7 @@ In summary, environment variables used by a formula need to conform to these fil
 
 ## Updating formulae
 
-Eventually a new version of the software will be released. In this case you should update the [`url`](https://rubydoc.brew.sh/Formula#url-class_method) and [`sha256`](https://rubydoc.brew.sh/Formula#sha256%3D-class_method). If a [`revision`](https://rubydoc.brew.sh/Formula#revision%3D-class_method) line exists outside any `bottle do` block *and* the new release is stable rather than devel, it should be removed.
+Eventually a new version of the software will be released. In this case you should update the [`url`](https://rubydoc.brew.sh/Formula#url-class_method) and [`sha256`](https://rubydoc.brew.sh/Formula#sha256%3D-class_method). If a [`revision`](https://rubydoc.brew.sh/Formula#revision%3D-class_method) line exists outside any `bottle do` block it should be removed.
 
 Leave the `bottle do ... end`  block as-is; our CI system will update it when we pull your change.
 

docs/How-To-Open-a-Homebrew-Pull-Request.md

@@ -63,7 +63,7 @@ To make a new branch and submit it for review, create a GitHub pull request with
     brew audit --strict <CHANGED_FORMULA>
     ```
 6. [Make a separate commit](Formula-Cookbook.md#commit) for each changed formula with `git add` and `git commit`.
-  * Please note that our preferred commit message format for simple version updates is "`<FORMULA_NAME> <NEW_VERSION>`", e.g. "`source-highlight 3.1.8`" but `devel` version updates should have the commit message suffixed with `(devel)`, e.g. "`nginx 1.9.1 (devel)`". If updating both `stable` and `devel`, the format should be a concatenation of these two forms, e.g. "`x264 r2699, r2705 (devel)`".
+  * Please note that our preferred commit message format for simple version updates is "`<FORMULA_NAME> <NEW_VERSION>`", e.g. "`source-highlight 3.1.8`".
 7. Upload your branch of new commits to your fork:
     ```sh
     git push --set-upstream <YOUR_USERNAME> <YOUR_BRANCH_NAME>