docs, errors: note RFC compliance in error message and documentation

added note and link to RFC7230

>A recipient MUST parse an HTTP message as a sequence of octets in an
encoding that is a superset of US-ASCII [USASCII].

Ref: https://tools.ietf.org/html/rfc7230#section-3

Ref: nodejs#18118
Ref: nodejs#18178
Ref: nodejs#19344

doc/api/errors.md

@@ -798,7 +798,7 @@ An attempt was made to add more headers after the headers had already been sent.
 ### ERR_HTTP_INCOMING_SOCKET_ENCODING
 
 An attempt was made to manipulate the encoding of an incoming request packet.
-This is not allowed (RFC2616).
+This is forbidden by [RFC7230 Section 3](https://tools.ietf.org/html/rfc7230#section-3)
 
 <a id="ERR_HTTP_INVALID_HEADER_VALUE"></a>
 ### ERR_HTTP_INVALID_HEADER_VALUE

lib/internal/errors.js

@@ -763,7 +763,7 @@ E('ERR_HTTP2_UNSUPPORTED_PROTOCOL', 'protocol "%s" is unsupported.', Error);
 E('ERR_HTTP_HEADERS_SENT',
   'Cannot %s headers after they are sent to the client', Error);
 E('ERR_HTTP_INCOMING_SOCKET_ENCODING',
-  'Incoming socket encoding is not allowed (RFC 2616)',
+  'Changing the incoming socket encoding is not possible (see RFC7230 Section 3)',
   Error);
 E('ERR_HTTP_INVALID_HEADER_VALUE',
   'Invalid value "%s" for header "%s"', TypeError);