Try to clarify extended uses of SSLCertificateFile.

Backport of r1682923 and r1682937 from trunk,
resp. r1682929 and r1682939 from 2.4.x.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@1682942 13f79535-47bb-0310-9956-ffa450edef68