lint-history: explain filename paths

It was not clear for some users that the filenames would be relative
paths from the toplevel of the repository.  Add some text to explain
this.

Reported-by: @ruv on GitHub
Signed-off-by: Elijah Newren <newren@gmail.com>

contrib/filter-repo-demos/lint-history

@@ -59,6 +59,9 @@ example_text = '''CALLBACK
         def is_relevant(filename):
             BODY
 
+    Where filename is the full relative path from the toplevel of the
+    repository.
+
     Thus, to only run on files with a ".txt" extension you would run
         lint-history --relevant 'return filename.endswith(b".txt")' ...