Fixes#3146.
This PR changes the Matrix Spec Proposals page to clarify that implementations **do not** need to wait until a spec release to use stable prefixes, but that they can do so after the corresponding MSC has been merged. The justification is that once an MSC has been merged, it's fairly guaranteed that it will land in the spec. Yet it will take time for the spec release process to run its course, and we shouldn't make implementations wait for that.
The exception to this is if implementating a feature in a backwards-compatible way requires a new spec version to indicate to clients/servers that a feature has been added/changed. This situation is rare though, and most implementations won't fall into this category.
It wasn't entirely clear what should happen to the FCP timer (and state) when a concern is raised during FCP. After some discussion, we agreed that when a concern is raised:
1. FCP will continue to not conclude until at least 5 days have passed, but once those 5 days are up it *still* won't conclude until all concerns raised during FCP are resolved.
2. If a concern warrants a large enough change in the document, then the Spec Core Team may consider cancelling FCP and restarting the timer in order for people to have some time to think about and review the new changes.
