Document the actual current implementation requirements of the MSC process

**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.

Start with the MSC process documentation at https://spec.matrix.org/proposals/ and compare its stated implementation timing with the process described in the issue. Update the documentation so it matches the current practice, including the implementation requirement before FCP, and verify that the process description is consistent throughout.

Written by the indexing model from the issue text.