Mention Google OAuth API change in update guides

Add oauth doc to 6.0 to 7.2 upgrader

doc/update/6.0-to-7.2.md

@@ -129,6 +129,12 @@ To make sure you didn't miss anything run a more thorough check with:
 
 If all items are green, then congratulations upgrade complete!
 
+## 10. Update OmniAuth configuration
+
+When using Google omniauth login, changes of the Google account required.
+Ensure that `Contacts API` and the `Google+ API` are enabled in the [Google Developers Console](https://console.developers.google.com/).
+More details can be found at the [integration documentation](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/integration/google.md).
+
 ## Things went south? Revert to previous version (6.0)
 
 ### 1. Revert the code to the previous version

doc/update/7.1-to-7.2.md

@@ -91,6 +91,12 @@ To make sure you didn't miss anything run a more thorough check with:
 
 If all items are green, then congratulations upgrade is complete!
 
+### 8. Update OmniAuth configuration
+
+When using Google omniauth login, changes of the Google account required.
+Ensure that `Contacts API` and the `Google+ API` are enabled in the [Google Developers Console](https://console.developers.google.com/).
+More details can be found at the [integration documentation](../integration/google.md).
+
 ## Things went south? Revert to previous version (7.1)
 
 ### 1. Revert the code to the previous version