doc: add http2 and http3 limitation doc comment for ngx.req.socket API.

Author: oowl

README.markdown

@@ -5643,6 +5643,8 @@ Returns a read-only cosocket object that wraps the downstream connection. Only [
 
 In case of error, `nil` will be returned as well as a string describing the error.
 
+Due to the streaming nature of HTTP2 and HTTP3, this API cannot be used when the downstream connection is HTTP2 and HTTP3.
+
 The socket object returned by this method is usually used to read the current request's body in a streaming fashion. Do not turn on the [lua_need_request_body](#lua_need_request_body) directive, and do not mix this call with [ngx.req.read_body](#ngxreqread_body) and [ngx.req.discard_body](#ngxreqdiscard_body).
 
 If any request body data has been pre-read into the Nginx core request header buffer, the resulting cosocket object will take care of this to avoid potential data loss resulting from such pre-reading.

doc/HttpLuaModule.wiki

@@ -4741,6 +4741,8 @@ Returns a read-only cosocket object that wraps the downstream connection. Only [
 
 In case of error, <code>nil</code> will be returned as well as a string describing the error.
 
+Due to the streaming nature of HTTP2 and HTTP3, this API cannot be used when the downstream connection is HTTP2 and HTTP3.
+
 The socket object returned by this method is usually used to read the current request's body in a streaming fashion. Do not turn on the [[#lua_need_request_body|lua_need_request_body]] directive, and do not mix this call with [[#ngx.req.read_body|ngx.req.read_body]] and [[#ngx.req.discard_body|ngx.req.discard_body]].
 
 If any request body data has been pre-read into the Nginx core request header buffer, the resulting cosocket object will take care of this to avoid potential data loss resulting from such pre-reading.