http2: Docs warning against direct socket access (Squash)

Author: sebdeckers

doc/api/http2.md

@@ -227,6 +227,8 @@ process has completed.
 A reference to the [`net.Socket`][] or [`tls.TLSSocket`][] to which this
 `Http2Session` instance is bound.
 
+Directly reading from and writing to the socket is strongly discouraged.
+
 #### http2session.state
 
 * Value: {Object}