Minor fixes in ab docs.

Backport of r1307790,1308327,1308459,1309536,
1331942,1331977 from trunk.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1362120 13f79535-47bb-0310-9956-ffa450edef68