DOC: config: Add notes about errors emitted by H1 mux

DOC: config: Add notes about errors emitted by H1 mux

Now, some errors are handled by the H1 multiplexer. During the headers
parsing request, there is no stream attached to the H1 mux. Thus, if an
error is reported at this stage, it is handled by the mux itself. If
possible the corresponding frontend errorfile is used, but it should be a
static message. Custom error messages are not supported. Otherwise, default
error messages are used.

In addition, the http analysis has not started yet, so http-after-response
ruleset is not evaluated and cannot alter these early responses.