Merge r1902117, r1902130, r1902132, r1902133 from trunk:

Merge r1902117, r1902130, r1902132, r1902133 from trunk:

*) mod_heartmonitor: Allow "HeartbeatMaxServers 0"
   to use file based storage instead of slotmem.
   Needed after setting HeartbeatMaxServers default
   to the documented value 10 in 2.4.54.
   [Jérôme Billiras]

Fix numeric check for HeartbeatMaxServers.

*) mod_heartmonitor: Document interaction between
   HeartbeatStorage and HeartbeatMaxServers.

Fix new typo in docs...

Submitted by: rjung
Reviewed by: rjung, rpluem, jim

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1903619 13f79535-47bb-0310-9956-ffa450edef68