Clarified trac patch-related field meaning

There was some confusion on the mentorship list as to whether 'needs tests'
meant that a given ticket might not need tests, as opposed to flagging
a specific patch as still needing tests.

This commit seeks to clarify that confusion.

docs/internals/contributing/triaging-tickets.txt

@@ -175,6 +175,9 @@ This means the ticket has an associated
 :doc:`patch<writing-code/submitting-patches>`. These will be reviewed
 to see if the patch is "good".
 
+The following three fields (Needs documentation, Needs tests, 
+Patch needs improvement) apply only if a patch has been supplied.
+
 Needs documentation
 ~~~~~~~~~~~~~~~~~~~