Add compatibility note for the 3rd argument of 'RewriteMap'

(backported in 2.4.x in 1811748)

Improve doc highlight

(r1827603 and r1827605) in trunk

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1827607 13f79535-47bb-0310-9956-ffa450edef68