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

* added doc fragment to multiple network modules

lib/ansible/modules/network/system/net_banner.py

@@ -21,6 +21,7 @@ description:
   - This will configure both login and motd banners on network devices.
     It allows playbooks to add or remove
     banner text from the active running configuration.
+extends_documentation_fragment: network_agnostic
 options:
   banner:
     description:

lib/ansible/modules/network/system/net_logging.py

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

lib/ansible/modules/network/system/net_ping.py

@@ -23,6 +23,7 @@ description:
   - Tests reachability using ping from network device to a remote destination.
   - For Windows targets, use the M(win_ping) module instead.
   - For targets running Python, use the M(ping) module instead.
+extends_documentation_fragment: network_agnostic
 options:
   count:
     description:

lib/ansible/modules/network/system/net_system.py

@@ -24,6 +24,7 @@ description:
     on network devices.  It provides an option to configure host system
     parameters or remove those parameters from the device active
     configuration.
+extends_documentation_fragment: network_agnostic
 options:
   hostname:
     description:

lib/ansible/modules/network/system/net_user.py

@@ -24,6 +24,7 @@ description:
     either individual usernames or the aggregate of usernames in the
     current running config. It also supports purging usernames from the
     configuration that are not explicitly defined.
+extends_documentation_fragment: network_agnostic
 options:
   aggregate:
     description: