tests: add a comment in a test that will hopefully save someone some time

I spent at least an hour, probably closer to 1.5, trying to figure out what this
was complaining about. Hopefully anyone else in my position will see this note
and not waste the time.

Differential Revision: https://phab.mercurial-scm.org/D9953

tests/test-check-module-imports.t

@@ -14,6 +14,10 b' these may expose other cycles.'
 Known-bad files are excluded by -X as some of them would produce unstable
 outputs, which should be fixed later.
 
+NOTE: the `hg locate` command here only works on files that are known to
+Mercurial. If you add an import of a new file and haven't yet `hg add`ed it, you
+will likely receive warnings about a direct import.
+
   $ testrepohg locate 'set:**.py or grep(r"^#!.*?python")' \
   > 'tests/**.t' \
   > -X hgweb.cgi \