e2ee/device verification: clarifications

[sumnerevans]

• e2ee/device verification: move general error handling under framework

  Most of the "Error and exception handling" section is generally applicable to other verification methods besides SAS, so I moved those bullet points under "Key verification framework".

• e2ee/device verification/error handling: fix typo start -> request

  It used to be possible to create a start request without a request, but this was deprecated.

• m.key.verification.start: remove wording that it is typically to-device

  It can be either a to-device event or an in-room-event.

• e2ee/device verification: normalize all links

  Changes the format of all links in the Device Verification section to be:

  [`m.key.verification.whatever`](#mkeyverificationwhatever)

  This involved adding links in a bunch of places, but also changing many links that did not have code within them to have the backticks.

• e2ee/device verification start: clarify required nature of transaction_id or m.relates_to

  Add notes about the fact that the start event may be the first event sent during a verification process and that clients should handle other clients doing so, but not themselves send the start event first.

Signed-off-by: Sumner Evans sumner.evans@automattic.com

Pull Request Checklist

• [x] Pull request includes a changelog file
• [x] Pull request includes a sign off
• [x] Pull request is classified as 'other changes'

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[richvdh]

Suddenly there's an awful lot changing in this PR, and it's quite hard to review...