Clarify the ordering and constraints on `chain::Confirm`

[TheBlueMatt]

They're really not well-phrased right now. We should highlight a few things:

• "Should be called when a new header is available but may be skipped for intermediary blocks if they become available at the same time." on best_block_updated really means "whenever you poll, call this" not "if the block is within 1 second of the old one you can skip",
• Focus the ordering section on the things that matter: transactions topologically sorted and things strictly ordered only in case of reorgs.

[gabbyprecious]

Hello @TheBlueMatt Can I work on this?

[TheBlueMatt]

This may be kinda tough for a new contributor, maybe take a look at issues with the good first issue tag.

[tnull]

Will pick this up.