docs(messaging): Document the 8 second delay introduced by the earlier mitigation to Issue 4144

[brianathere]

Description

This change documents the 8 second delay in the delivery of background notifications so developers are aware of the minimum time it will take for a notification do make it to the application in a background state and not have to wonder why it's taking so long.

Related issues

4144

Release Summary

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
  • Yes
• My change supports the following platforms;
  • Android
  • iOS
• My change includes tests;
  • e2e tests added or updated in packages/\*\*/e2e
  • jest tests added or updated in packages/\*\*/__tests__
• I have updated TypeScript types that are affected by my change.
• This is a breaking change;
  • Yes
  • [X ] No

Test Plan

Documentation change

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[mikehardy]

I see you have a maybe-actually-works-around-the-issue PR separately, but this is a nice docs change for now, the delay can be unexpected, that has come up before, thank you!