[IMPROV] PageTitleFilterPageGenerator: docstring more specific

The docstring explicitly says that the titles need to be regexable
strings. Also simplified the test if there is an entry for family/code
in ignore_list.

Change-Id: I1ef136bcd6d9d1cd2d891024b85dfed414b83eef
Merge "[IMPROV] PageTitleFilterPageGenerator: docstring more specific"