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

* added doc fragment to multiple network modules

lib/ansible/modules/network/layer2/net_l2_interface.py

@@ -21,6 +21,7 @@ short_description: Manage Layer-2 interface on network devices
 description:
   - This module provides declarative management of Layer-2 interface
     on network devices.
+extends_documentation_fragment: network_agnostic
 options:
   name:
     description:

lib/ansible/modules/network/layer2/net_vlan.py

@@ -22,6 +22,7 @@ short_description: Manage VLANs on network devices
 description:
   - This module provides declarative management of VLANs
     on network devices.
+extends_documentation_fragment: network_agnostic
 options:
   name:
     description: