Merge branch 'sh-troubleshooting-openid-docs' into 'master'

Add OpenID Connect troubleshooting docs

See merge request gitlab-org/gitlab-ce!29379

doc/administration/auth/oidc.md

@@ -138,3 +138,26 @@ for more details:
    }
  }
 ```
+
+### Troubleshooting
+
+If you're having trouble, here are some tips:
+
+1. Ensure `discovery` is set to `true`. Setting it to `false` requires
+specifying all the URLs and keys required to make OpenID work.
+
+1. Check your system clock to ensure the time is synchronized properly.
+
+1. As mentioned in [the
+documentation](https://github.com/m0n9oose/omniauth_openid_connect),
+make sure `issuer` corresponds to the base URL of the Discovery URL. For
+example, `https://accounts.google.com` is used for the URL
+`https://accounts.google.com/.well-known/openid-configuration`.
+
+1. The OpenID Connect client uses HTTP Basic Authentication to send the
+OAuth2 access token. For example, if you are seeing 401 errors upon
+retrieving the `userinfo` endpoint, you may want to check your OpenID
+Web server configuration. For example, for
+[oauth2-server-php](https://github.com/bshaffer/oauth2-server-php), you
+may need to [add a configuration parameter to
+Apache](https://github.com/bshaffer/oauth2-server-php/issues/926#issuecomment-387502778).