Update sync testing docs.

[tublitzed]

We started adding to the docs here and it looks like a few things have happened since then that are worth documenting to save folks time in the future:

1. Testing Fenix against both staging and self-hosted sync.
2. Building iOS locally pointed to local Durable sync.

^ In at least the case of #2, it's worth making sure we write this up somewhere as it's time consuming to figure out otherwise.

[jrconlin]

A few notes on getting fenix working with a custom server:
(Note: this is a work in progress, additional info welcome)

1. You must not be connected to a sync server already. If you are, log out of sync.
2. Go to the Settings Kebab menu:

• Settings
• About Firefox...
• Tap on the Firefox logo until developer options enabled (5 times?)
  This should enable several new menu options, including
  Custom Firefox Accounts Server and Custom Sync Server

[Silvenga]

Is there a minimum version that supports this under Android? Per mozilla-mobile/fenix#3762 (comment) and my own experences, support for self-hosted sync is broken on the latest stable for Android.

[tublitzed]

The staging docs are in review here but I'm going to keep this issue open after it merges to make sure we don't forget to come back and add details for self-hosted sync as well as custom local builds.