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

    have to be present in the server.</p>

    <p><module>mod_proxy_http2</module> works with incoming requests

    over HTTP/1.1 and HTTP/2 requests. If <module>mod_http2</module>

    handles the frontend connection, requests against the same HTTP/2

    backend are sent over a single connection, whenever possible.</p>

    over HTTP/1.1 and HTTP/2 requests. In both cases, requests proxied

    to the same backend are sent over a single connection

    whenever possible (namely when the connection can be re-used).</p>

    <p><module>mod_proxy_http2</module> will not use the HTTP/2 protocol

    when the frontend requests use HTTP/1.1.

    This means that HTTP/2 will be used to proxy requests to a capable backend

    only when the frontend requests use the same protocol.</p>

    <p>This module relies on <a href="http://nghttp2.org/">libnghttp2</a>

    to provide the core http/2 engine.</p>