lib-http: Document why client response parser is lenient

lib-http: Document why client response parser is lenient

Record why http_response_parser_init() is intentionally not passed
STRICT here: the client parses responses from arbitrary upstream
servers, some of which send legitimately non-compliant framing, and
the response is consumed internally rather than re-emitted
downstream, so the response-splitting precondition that STRICT
guards against doesn't apply to this connection's use.

No behavior change.