Clarify that Password authentication must be enabled for OAuth flow

Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/454286

**Problem**

Resource owner password credentials flow doesn't work when Password
authencation option is disabled for the GitLab instance.

**Solution**

Add a note to the documentation that explains that.

doc/api/oauth2.md

@@ -272,6 +272,10 @@ The Resource Owner Password Credentials is disabled for users with
 These users can access the API using [personal access tokens](../user/profile/personal_access_tokens.md)
 instead.
 
+NOTE:
+Ensure the [**Allow password authentication for Git over HTTP(S)**](../administration/settings/sign_in_restrictions.md#password-authentication-enabled)
+checkbox is selected for the GitLab instance to support the password credentials flow.
+
 In this flow, a token is requested in exchange for the resource owner credentials
 (username and password).