Document is_valid_email() as deprecated.

Its merely just a wrapper now.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5619 2fea6a4d-a838-0410-917b-93a53c48e9d2
Update email package name and some examples.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5620 2fea6a4d-a838-0410-917b-93a53c48e9d2