Update HOMEBREW_DEBUG in man page.

Closes Homebrew/homebrew#17344.

Library/Contributions/manpages/brew.1.md

@@ -435,7 +435,7 @@ can take several different forms:
     If set, Homebrew will pass `--verbose` when invoking `curl`(1).
 
   * HOMEBREW\_DEBUG:
-    If set, Homebrew always assumes `--debug` when running commands.
+    If set, any commands that can emit debugging information will do so.
 
   * HOMEBREW\_DEBUG\_INSTALL:
     When `brew install -d` or `brew install -i` drops into a shell,

share/man/man1/brew.1

@@ -478,7 +478,7 @@ If set, Homebrew will pass \fB\-\-verbose\fR when invoking \fBcurl\fR(1)\.
 .
 .TP
 HOMEBREW_DEBUG
-If set, Homebrew always assumes \fB\-\-debug\fR when running commands\.
+If set, any commands that can emit debugging information will do so\.
 .
 .TP
 HOMEBREW_DEBUG_INSTALL