doc: document the grpc-port option, now cln-grpc is a built-in.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

doc/lightningd-config.5.md

@@ -220,6 +220,11 @@ If there is no `hsm_secret` yet, `lightningd` will create a new encrypted secret
 If you have an unencrypted `hsm_secret` you want to encrypt on-disk, or vice versa,
 see lightning-hsmtool(8).
 
+ **grpc-port**=*portnum* [plugin `cln-grpc`]
+
+The port number for the GRPC plugin to listen for incoming
+connections; default is not to activate the plugin at all.
+
 ### Lightning node customization options
 
  **alias**=*NAME*