docs: fix documentation of rmcp client flag (#7665)

## Summary
- Updated the rmcp client flag's documentation in config.md file
- changed it from `experimental_use_rmcp_client` to `rmcp_client`

Author: JaySabva

docs/config.md

@@ -464,10 +464,11 @@ http_headers = { "HEADER_NAME" = "HEADER_VALUE" }
 env_http_headers = { "HEADER_NAME" = "ENV_VAR" }
 ```
 
-Streamable HTTP connections always use the experimental Rust MCP client under the hood, so expect occasional rough edges. OAuth login flows are gated on the `experimental_use_rmcp_client = true` flag:
+Streamable HTTP connections always use the experimental Rust MCP client under the hood, so expect occasional rough edges. OAuth login flows are gated on the `rmcp_client = true` flag:
 
 ```toml
-experimental_use_rmcp_client = true
+[features]
+rmcp_client = true
 ```
 
 After enabling it, run `codex mcp login <server-name>` when the server supports OAuth.
@@ -489,17 +490,6 @@ disabled_tools = ["search"]
 
 When both `enabled_tools` and `disabled_tools` are specified, Codex first restricts the server to the allow-list and then removes any tools that appear in the deny-list.
 
-#### Experimental RMCP client
-
-This flag enables OAuth support for streamable HTTP servers.
-
-```toml
-experimental_use_rmcp_client = true
-
-[mcp_servers.server_name]
-…
-```
-
 #### MCP CLI commands
 
 ```shell