Add note on track category used for UA

Adds a note about the device-mode logic for setting category to track calls for Google Analytics UA.
https://github.com/segmentio/analytics.js-integrations/blob/master/integrations/google-analytics/lib/index.js#L313

Author: tobiordobi

src/connections/destinations/catalog/google-analytics/index.md

@@ -195,6 +195,8 @@ For this example these event attributes are sent to Google Universal Analytics:
 | **Event Category** | All       |
 | **Event Action**   | Logged In |
 
+> info ""
+> **Note**: In device-mode only, if you pass `category` to the [`page`](https://segment.com/docs/connections/destinations/catalog/google-analytics/#page-and-screen) call, we will use the `category` from `page` instead of setting default **Event Category** to `All`.
 
 And another Track call example, this time with all Google Universal Analytics event parameters: