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".

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

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

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

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

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

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

…amework

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

…amework

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

…amework

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

…amework

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

…amework

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

…amework

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

…amework

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

…amework

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

This commit 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.

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

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

…n_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>

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