Update documentation for SafeMode and TxPause Pallets (#2413)

# Description

Documentation for the TxPause and SafeMode pallets.

Based on reading and the notes from the following related PRs:
- https://github.com/paritytech/substrate/pull/12092



I believe this also completes the checklist to be able to close the
related PRs:
- Close #302 
- Close #274

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Co-authored-by: joe petrowski <[email protected]>