[HttpClient] Document AMPHP HttpClient integration for HTTP/2

[wouterj]

Fixes #13293

Just below the shown diff, there is this paragraph:

Support for HTTP/2 PUSH works out of the box when libcurl >= 7.61 is used with
PHP >= 7.2.17 / 7.3.4: pushed responses are put into a temporary cache and are
used when a subsequent request is triggered for the corresponding URLs.

Should AMPHP also be added here?

[javiereguiluz]

Thanks Wouter.

[javiereguiluz]

Note that I didn't update the paragraph you mentioned while merging ... because it was not clear if we should update it or not. We can do that in a different PR. Thanks.