Push activation documentation has confusing library instance example & lacks details on clientId

[mattheworiordan]

This code example doesn't explain:

• Where a getAblyRealtime() comes from, why are we not using the official Ably.Realtime constructor
• How auth works in the context of an activate call
• It is very unclearly how a clientId is assigned to a device on activation. @paddybyers is making some changes to how this works anyway, so we should update in due course with that change rolling out.

┆Issue is synchronized with this Jira Bug by Unito