Merge r1779578, r1779609 from trunk:

Added more details to mod-proxy-http2's doc

Reworked mod-proxy-http2's documentation after Stefan's suggestions

Submitted by: elukey


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1780044 13f79535-47bb-0310-9956-ffa450edef68