….sas.v1* variant.

Currently the *m.key.verification.start* event appears twice with the
exact same title, in the "Key verification framework" section and the
"Short Authentication (SAS) verification" section. It's not immediately
clear that the first occurrence describes the format of the event in
general terms and that the second occurrence describes the fields when
the *m.sas.v1* verification method is being used. This is a similar
relationship to the *m.room.message* event and its various *msgtype*
variants.

This commit does three things:

* It tweaks the generation of the documentation to change the title
  of the second occurrence of *m.key.verification.start* to
  distinguish it from the first.
* It updates the language in the description of the two versions of the
  event to better describe the relationship between the two.
* It adds the optional `next_method` field to the schema of the
  *m.sas.v1* variant, as specified in the general form of
  *m.key.verification.start*.

Signed-off-by: Jimmy Cuadra <jimmy@jimmycuadra.com>

Signed-off-by: Jimmy Cuadra <jimmy@jimmycuadra.com>

* Switch "an SAS" back to "a SAS"
* Remove the `next_method` field from m.key.verification.start$m.sas.v1
  but add additional clarification to its description on
  m.key.verification.start that it is never present for methods that
  verify keys both ways.