manpage: note env files do not support expansion.

Fixes #16027

Library/Homebrew/manpages/brew.1.md.erb

@@ -162,6 +162,9 @@ User-specific environment files take precedence over prefix-specific files and
 prefix-specific files take precedence over system-wide files (unless
 `HOMEBREW_SYSTEM_ENV_TAKES_PRIORITY` is set, see below).
 
+Note that these files do not support shell variable expansion e.g. `$HOME` or
+command execution e.g. `$(cat file)`.
+
 <%= environment_variables %>
 
 ## USING HOMEBREW BEHIND A PROXY

docs/Manpage.md

@@ -2080,6 +2080,9 @@ User-specific environment files take precedence over prefix-specific files and
 prefix-specific files take precedence over system-wide files (unless
 `HOMEBREW_SYSTEM_ENV_TAKES_PRIORITY` is set, see below).
 
+Note that these files do not support shell variable expansion e.g. `$HOME` or
+command execution e.g. `$(cat file)`.
+
 - `HOMEBREW_API_DOMAIN`
   <br>Use this URL as the download mirror for Homebrew JSON API. If metadata files at that URL are temporarily unavailable, the default API domain will be used as a fallback mirror.
 

manpages/brew.1

@@ -2956,6 +2956,9 @@ Note that environment variables must have a value set to be detected\. For examp
 .P
 User\-specific environment files take precedence over prefix\-specific files and prefix\-specific files take precedence over system\-wide files (unless \fBHOMEBREW_SYSTEM_ENV_TAKES_PRIORITY\fR is set, see below)\.
 .
+.P
+Note that these files do not support shell variable expansion e\.g\. \fB$HOME\fR or command execution e\.g\. \fB$(cat file)\fR\.
+.
 .TP
 \fBHOMEBREW_API_DOMAIN\fR
 .