Add note to network agnostic modules for supported platforms (#51617)

* added doc fragment to multiple network modules

lib/ansible/modules/network/netconf/netconf_config.py

@@ -23,7 +23,9 @@ description:
       the IETF. It is documented in RFC 6241.
     - This module allows the user to send a configuration XML file to a netconf
       device, and detects if there was a configuration change.
-extends_documentation_fragment: netconf
+extends_documentation_fragment:
+    - netconf
+    - network_agnostic
 options:
   content:
     description:

lib/ansible/modules/network/netconf/netconf_get.py

@@ -26,6 +26,7 @@ description:
       the IETF. It is documented in RFC 6241.
     - This module allows the user to fetch configuration and state data from NETCONF
       enabled network devices.
+extends_documentation_fragment: network_agnostic
 options:
   source:
     description:

lib/ansible/modules/network/netconf/netconf_rpc.py

@@ -26,6 +26,7 @@ description:
       the IETF. It is documented in RFC 6241.
     - This module allows the user to execute NETCONF RPC requests as defined
       by IETF RFC standards as well as proprietary requests.
+extends_documentation_fragment: network_agnostic
 options:
   rpc:
     description: