Some users require hands to be held constantly

Tell user exactly what command to use rather than thinking they'll figure it out by applying some negation logic to the main verb in the sentence they just read.

Closes Homebrew/homebrew#12790.

Library/Homebrew/cmd/doctor.rb

@@ -897,10 +897,14 @@ def check_for_unlinked_but_not_keg_only
     end
   end.map{ |pn| pn.basename }
 
+  # NOTE very old kegs will be linked without the LinkedKegs symlink
+  # this will trigger this warning but it's wrong, we could detect that though
+  # but I don't feel like writing the code.
+
   if not unlinked.empty? then <<-EOS.undent
     You have unlinked kegs in your Cellar
     Leaving kegs unlinked can lead to build-trouble and cause brews that depend on
-    those kegs to fail to run properly once built.
+    those kegs to fail to run properly once built. Run `brew link` on these:
 
         #{unlinked * "\n        "}
     EOS