clarify when regexps are case-sensitive

There are numerous bug reports about case sensitivity
in various parts of Mailman. Here I request general
clarification/documentation in the web admin's
interface of case sensitivity for the regexp variables
accessible via the administrative interface
(header_filter_rules, etc.). If all regexps are
treated the same, let's please have the default
explained in a good place (even if it's just to say
that Python defaults apply). When individual variables
differ from the default, the details should be there in
the variable's documentation. Example: as I consider
how to set up my privacy filters (header_filter_rules,
etc.) I realize I will have to experiment to see
whether I need to deal with upper/lowercase issues in
my regexp values, because I can find no explanation of
how Mailman deals with case.

[http://sourceforge.net/tracker/index.php?func=detail&aid=1387349&group_id=103&atid=350103]