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

* added doc fragment to multiple network modules

lib/ansible/plugins/doc_fragments/network_agnostic.py

@@ -0,0 +1,27 @@
+# Copyright (c) 2019 Ansible, Inc
+#
+# This file is part of Ansible
+#
+# Ansible is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# Ansible is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with Ansible.  If not, see <http://www.gnu.org/licenses/>.
+
+
+class ModuleDocFragment(object):
+
+    # Standard files documentation fragment
+    DOCUMENTATION = '''
+options: {}
+notes:
+  - This module is supported on C(ansible_network_os) network platforms. See
+    the :ref:`Network Platform Options <platform_options>` for details.
+'''