[TF-7737] Update documentation on how Branch and Tags effect PublishingMechanism

Author: hashimoon

registry_module.go

@@ -270,6 +270,11 @@ type RegistryModuleUpdateOptions struct {
 	// **Note: This field is still in BETA and subject to change.**
 	TestConfig *RegistryModuleTestConfigOptions `jsonapi:"attr,test-config,omitempty"`
 
+	// The Branch and Tag fields are used to determine
+	// the PublishingMechanism for a RegistryModule that has a VCS a connection.
+	// When a value for Branch is provided, the Tags field is removed on the server
+	// When a value for Tags is provided, the Branch field is removed on the server
+	// **Note: This field is still in BETA and subject to change.**
 	Branch string `jsonapi:"attr,branch,omitempty"`
 	Tags   string `jsonapi:"attr,tags,omitempty"`
 }