commands: explain change in command description splitting

A comment here would help the reader to understand the need for this
splitting logic, which is not so straightforward.

Addresses review comment in
https://github.com/Homebrew/brew/pull/15146#discussion_r1157361656.

Signed-off-by: Ruoyu Zhong <zhongruoyu@outlook.com>

Library/Homebrew/commands.rb

@@ -203,6 +203,10 @@ module Commands
 
     if (cmd_parser = Homebrew::CLI::Parser.from_cmd_path(path))
       if short
+        # We only consider a dot as a full stop if it is either followed by a
+        # whitespace or at the end of the description. In this way we can
+        # prevent cutting off a sentence in the middle due to dots in URLs or
+        # paths.
         cmd_parser.description.split(/\.(?>\s|$)/).first
       else
         cmd_parser.description