Add documentation for TargetGroupName (#1366)

elb_application_lb/elb_network_lb - Add documentation for TargetGroupName

SUMMARY
fixes: #915
elb_application_lb and elb_network_lb have a poorly documented feature, that you can use TargetGroupName instead of TargetGroupArn.  While this is shown in the examples, it's in the options documentation.
While undocumented the feature's been there since at release 1.0.0
ISSUE TYPE

Docs Pull Request

COMPONENT NAME
plugins/modules/elb_application_lb.py
plugins/modules/elb_network_lb.py
ADDITIONAL INFORMATION

Reviewed-by: Alina Buzachis <None>

Author: tremble

plugins/modules/elb_application_lb.py

@@ -122,8 +122,14 @@
                     description: The type of action.
                     type: str
                 TargetGroupArn:
-                    description: The Amazon Resource Name (ARN) of the target group.
+                    description:
+                      - The Amazon Resource Name (ARN) of the target group.
+                      - Mutually exclusive with I(TargetGroupName).
                     type: str
+                TargetGroupName:
+                    description:
+                      - The name of the target group.
+                      - Mutually exclusive with I(TargetGroupArn).
         Rules:
             type: list
             elements: dict

plugins/modules/elb_network_lb.py

@@ -64,8 +64,14 @@
                     description: The type of action.
                     type: str
                 TargetGroupArn:
-                    description: The Amazon Resource Name (ARN) of the target group.
+                    description:
+                      - The Amazon Resource Name (ARN) of the target group.
+                      - Mutually exclusive with I(TargetGroupName).
                     type: str
+                TargetGroupName:
+                    description:
+                      - The name of the target group.
+                      - Mutually exclusive with I(TargetGroupArn).
   name:
     description:
       - The name of the load balancer. This name must be unique within your AWS account, can have a maximum of 32 characters, must contain only alphanumeric