stable-4.x: Update vmware_guest.py docs (#2285)

SUMMARY
Only one of name or vlan is in fact required.
ISSUE TYPE

Docs Pull Request

COMPONENT NAME
vmware_guest
ADDITIONAL INFORMATION
Backport of #2272

plugins/modules/vmware_guest.py

@@ -526,13 +526,13 @@
             type: str
             description:
             - Name of the portgroup or distributed virtual portgroup for this interface.
-            - Required per entry.
+            - This is required if C(vlan) isn't defined.
             - When specifying distributed virtual portgroup make sure given O(esxi_hostname) or O(cluster) is associated with it.
         vlan:
             type: int
             description:
             - VLAN number for this interface.
-            - Required per entry.
+            - This is required if C(name) isn't defined.
         device_type:
             type: str
             description: