Document the actual current implementation requirements of the MSC process

[HarHarLinks]

Link to problem area:
https://spec.matrix.org/proposals/

Issue
As far as I know, the actual process is to require an implementation before FCP.
According to documentation, the implementation is required only after FCP, before merging the spec PR.

Expected behaviour
Alignment of documentation and practice. Imo, favor the actual process.

[turt2live]

We changed the process to require implementation prior to FCP, but forgot to update large swathes of the documented process.

The change was made because we were finding that a lot of MSCs required changes after implementation actually happened, so wanted to frontload that for a more streamlined FCP review.

[richvdh]

https://spec.matrix.org/proposals/ is quite a big page. If there are particular bits which are wrong, it would be helpful to point them out

[turt2live]

About half the page talks about implementation post-FCP still.

https://spec.matrix.org/proposals/#process is wrong
https://spec.matrix.org/proposals/#lifetime-states for finished-final-comment-period is wrong
etc