os/mac: note installation docs need bumped too.

I noticed these contain a reference to our supported versions so should
be bumped whenever this version is bumped.

Library/Homebrew/os/mac.rb

@@ -35,12 +35,14 @@ module OS
     end
 
     def latest_stable_version
-      # TODO: bump version when new macOS is released
+      # TODO: bump version when new macOS is released and also update
+      # references in docs/Installation.md
       Version.new "10.13"
     end
 
     def outdated_release?
-      # TODO: bump version when new macOS is released
+      # TODO: bump version when new macOS is released and also update
+      # references in docs/Installation.md
       version < "10.11"
     end