Fix gh-5270: Clarify request_parse_body behavior with consumed body

[jordikroon]

Fixes #5270

[lacatoire]

Both code paths in urlencoded and multipart call sapi_module.read_post() with no guard against a previously consumed input stream. Once php://input has been read, the SAPI socket is drained and request_parse_body() gets nothing.

Two concerns with the current wording:

1. "may no longer be available", the behavior is deterministic, not conditional. If the body was consumed, request_parse_body() will return empty arrays.
2. "the raw input stream", is a bit vague. What's gone is the body data, not the stream itself.

a way to cover both would be:

  <caution>
   <simpara>
    The request body can only be consumed once.
    <function>request_parse_body</function> consumes the request body without
    buffering it to the <literal>php://input</literal> stream.
    Conversely, if the body has already been read (e.g. via
    <literal>php://input</literal>), <function>request_parse_body</function>
    will return empty data.
   </simpara>
  </caution>