Crashlytics iOS Documentation Issue [Legacy/New Build System, Last Build Script]

### Problem Description:
Crashlytics documentation has a note in step 3 about fact that we need to be sure that the new run script will be run as the last build script.

**Step 3: Initialize Crashlytics**
**_Note: Make sure your new build phase is the project's last build phase or Crashlytics can't initialize properly._**
https://firebase.google.com/docs/crashlytics/get-started

We can guarantee this only with the **Legacy Build System**, but on **New** Build System this phase may not necessarily be the last one.
https://medium.com/xcblog/five-things-you-must-know-about-xcode-10-new-build-system-41676cd5fd6c

### Conclusion:

If you have already updated the code and it works correctly on the New Build System, please let us know and update the documentation.

If the code does not work correctly on a New Build System, please indicate in the documentation that only the Legacy Build System should be used, since only in it we can guarantee that the script will be executed last.