doc: Update manpages readme to current practice

This hasn't been accurate since #1860 and #5650.

manpages/README.md

 # Manual pages
 
-This directory contains the generated Homebrew man pages from the `brew man` command. This command creates the output from `#:` comments in files, sections extracted from the repository's `README.md` and `brew.1.md.erb`.
+This directory contains the generated Homebrew man pages from the `brew man` command. This command creates the output from the `CLI::Parser` definitions in files, sections extracted from the repository's `README.md` and `brew.1.md.erb`.