Merge r1226603, r1226775 from trunk:

fill in the heartbeat modules' documentation.


add heartbeat to list of lb methods living in other modules.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1226940 13f79535-47bb-0310-9956-ffa450edef68