Update and fix subnet documentation

Author: cmeissner

docs/plugins/subnet_module.rst

@@ -773,6 +773,9 @@ Parameters
 
       :ansible-option-type:`string`
 
+      :ansible-option-versionadded:`added in 1.6.0 of codeaffen.phpipam`
+
+
       .. raw:: html
 
         </div>

plugins/modules/subnet.py

@@ -66,6 +66,7 @@
         description:
             - Layer-2 routing domain the vlan belongs to
             - If you have I(vlan) with same number in different routing domains it need to be set on desired value
+        version_added: 1.6.0
         type: str
         required: false
         default: default