grafana/grafana#13340 complete oauth doc

docs/sources/auth/generic-oauth.md

@@ -17,6 +17,9 @@ can find examples using Okta, BitBucket, OneLogin and Azure.
 
 This callback URL must match the full HTTP address that you use in your browser to access Grafana, but with the prefix path of `/login/generic_oauth`.
 
+You may have to set the `root_url` option of `[server]` for the callback URL to be 
+correct. For example in case you are serving Grafana behind a proxy.
+
 Example config:
 
 ```bash

docs/sources/auth/github.md

@@ -46,6 +46,9 @@ team_ids =
 allowed_organizations =
 ```
 
+You may have to set the `root_url` option of `[server]` for the callback URL to be 
+correct. For example in case you are serving Grafana behind a proxy.
+
 Restart the Grafana back-end. You should now see a GitHub login button
 on the login page. You can now login or sign up with your GitHub
 accounts.

docs/sources/auth/gitlab.md

@@ -58,6 +58,9 @@ api_url = https://gitlab.com/api/v4
 allowed_groups =
 ```
 
+You may have to set the `root_url` option of `[server]` for the callback URL to be 
+correct. For example in case you are serving Grafana behind a proxy.
+
 Restart the Grafana backend for your changes to take effect.
 
 If you use your own instance of GitLab instead of `gitlab.com`, adjust

docs/sources/auth/google.md

@@ -45,6 +45,9 @@ allowed_domains = mycompany.com mycompany.org
 allow_sign_up = true
 ```
 
+You may have to set the `root_url` option of `[server]` for the callback URL to be 
+correct. For example in case you are serving Grafana behind a proxy.
+
 Restart the Grafana back-end. You should now see a Google login button
 on the login page. You can now login or sign up with your Google
 accounts. The `allowed_domains` option is optional, and domains were separated by space.