diagnostic: detail TRAVIS opt-out reasoning.

Library/Homebrew/extend/os/mac/diagnostic.rb

@@ -53,7 +53,10 @@ module Homebrew
         return unless MacOS::Xcode.installed?
         return unless MacOS::Xcode.outdated?
 
-        # Travis CI images are going to end up outdated so don't complain.
+        # Travis CI images are going to end up outdated so don't complain when
+        # `brew test-bot` runs `brew doctor` in the CI for the Homebrew/brew 
+        # repository. This only needs to support whatever CI provided 
+        # Homebrew/brew is currently using.
         return if ENV["TRAVIS"]
 
         message = <<-EOS.undent