--prefix: move help to documentation comment

Library/Homebrew/cmd/--prefix.rb

+#:  * `--prefix`:
+#:    Display Homebrew's install path. *Default:* `/usr/local`
+#:
+#:  * `--prefix` <formula>:
+#:    Display the location in the cellar where <formula> is or would be installed.
+
 module Homebrew
   def __prefix
     if ARGV.named.empty?

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

@@ -50,12 +50,6 @@ With `--verbose` or `-v`, many commands print extra debugging information. Note
 
 <%= commands.join("\n") %>
 
-  * `--prefix`:
-    Display Homebrew's install path. *Default:* `/usr/local`
-
-  * `--prefix` <formula>:
-    Display the location in the cellar where <formula> is or would be installed.
-
   * `--repository`:
     Display where Homebrew's `.git` directory is located. For standard installs,
     the `prefix` and `repository` are the same directory.