Add documentation

docs/sources/auth/generic-oauth.md

@@ -209,6 +209,17 @@ allowed_organizations =
     token_url = https://<your domain>.my.centrify.com/OAuth2/Token/<Application ID>
     ```
 
+## Set up OAuth2 with non-compliant providers
+
+Some OAuth2 providers might not support `client_id` and `client_secret` passed via Basic Authentication HTTP header, which
+results in `invalid_client` error. To allow Grafana to authenticate via these type of providers, the client identifiers must be
+send via POST body, which can be enabled via the following settings:
+
+    ```bash
+    [auth.generic_oauth]
+    send_client_credentials_via_post = true
+    ```
+
 <hr>