Deprecate endTurn({ remove: true }) and document what numPlayers no longer means

[devill]

Two loose ends left by #1326 and #1327. Neither is urgent; both are cheap once the rest
lands.

endTurn({ remove: true })

Superseded by events.unseat(id), which removes a player through the same path as
leaveGame rather than as a side effect of ending a turn. Worth deprecating rather
than removing, since it is public API.

It also has a bug. In the arg.remove branch, flow.ts:582 returns the state
parameter rather than the locally updated one, so the updated ctx is discarded and
removing the last player silently does nothing. Present since #492 (2018).

concepts.md says nothing about what these mean now

After #1326, ctx.numPlayers is the seat count the match was created with — not how
many players are in it. It is deliberately left alone so that no existing game changes
behaviour, but that makes it the wrong thing to read in a game that opts into seating,
where it goes stale. ctx.players.length is the live count.

ctx.playOrder also needs a line: it is who plays this phase, in order, and it is
always a subset of ctx.players.