Consolidates advice about crafting regular expressions in one place,

Consolidates advice about crafting regular expressions in one place,
rather than having it duplicated in two docs. Gives example of using !
to negate a match, and notes that in the event of a not-match,
backreferences are not defined (because they didn't match) and points to
the backrefs section for further discussion.

Note that there's still probably a case to be made for a separate
regular expression reference, since neither `man perlre` nor Jeffrey's book
are likely to be in the possession of our audience.

(References patch on bz#70024 but takes it rather a different direction,
since that patch was not accurate.)

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1934224 13f79535-47bb-0310-9956-ffa450edef68