Add compatibility note for 'RegisterHttpMethod'

(backported in 2.4.x in 1772678)

(similar to r1827610 in trunk)

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1827611 13f79535-47bb-0310-9956-ffa450edef68