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

<identifier>proxy_http2_module</identifier>

<summary>

    <p>This module <em>requires</em> the service of <module

    >mod_proxy</module>. It provides the features used for

    proxying HTTP/2 requests. <module>mod_proxy_http2</module>

    supports HTTP/2 only. It does <em>not</em>

    provide any downgrades to HTTP/1.1.</p>

    <p><module>mod_proxy_http2</module>

    supports HTTP/2 only, it does <em>not</em>

    provide any downgrades to HTTP/1.1. This means that the backend

    needs to support HTTP/2 because HTTP/1.1 will not be used instead.</p>

    <p>Thus, in order to get the ability of handling HTTP/2 proxy requests,

    <p>This module <em>requires</em> the service of <module

    >mod_proxy</module>, so in order to get the ability of

    handling HTTP/2 proxy requests,

    <module>mod_proxy</module> and <module>mod_proxy_http2</module>

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

    need to be both loaded by the server.</p>

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

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

    to the same backend are sent over a single connection

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

    using HTTP/1.1 or HTTP/2. In both cases, requests proxied

    to the same backend are sent over a single TCP 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>Caveat: there will be no attemp to consolidate multiple HTTP/1.1 

    frontend requests (configured to be proxied to the same backend)

    into HTTP/2 streams belonging to the same HTTP/2 request.

    Each HTTP/1.1 frontend request will be proxied to the backend using

    a separate HTTP/2 request (trying to re-use the same TCP connection

    if possible).</p>

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

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