Add some clarifications around implementation requirements for MSCs (#1718)

* clarification around implementation requirement, and mention new label

* add changelog

* fix typo

* Fix typos

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

---------

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

changelogs/internal/newsfragments/1718.clarification

@@ -0,0 +1 @@
+Add some clarifications around implementation requirements for MSCs

content/proposals.md

@@ -380,9 +380,18 @@ As part of the proposal process the Spec Core Team will require evidence
 of the MSC working in order for it to move into FCP. This can usually be
 a branch/pull request to whichever implementation of choice that proves
 the MSC works in practice, though in some cases the MSC itself will be
-small enough to be considered proven. Where it's unclear if an MSC will
-require an implementation proof, ask in
-[\#matrix-spec:matrix.org](https://matrix.to/#/#matrix-spec:matrix.org).
+small enough to be considered proven. Implementations do not need to be
+merged or released, but must be of sufficient quality to show that the
+MSC works. Where it's unclear if an MSC will require an implementation
+proof, ask in [\#matrix-spec:matrix.org](https://matrix.to/#/#matrix-spec:matrix.org).
+Proposals may require both server-side and client-side implementations.
+
+Proposals that have not yet been implemented will have the
+`needs-implementation` label. After an implementation has been made, add a
+comment in the GitHub issue indicating so. After an implementation has been
+made, we will check it to verify that it implements the MSC. Proposals that
+have implementations that have not yet been checked will have the
+`implementation-needs-checking` label.
 
 ### Early release of an MSC/idea