Document endpoint /api/player/autocomplete (fix #847)

[ddugovic]

I am struggling to test this patch at the moment, however I think this should be close to correct.

[WandererXII]

/player/autocomplete is used in some places, so I think it would be better to keep that url.

[ddugovic]

OK... other lila-based sites use prefix /api for API endpoints, but Lishogi can be different for backward compatibility reasons.

[ddugovic]

Sorry... actually, could I update the "ui" references to this endpoint to use /api/player/autocomplete ? Long-term it may help both future lishogi development (copying code from lila) and help client API designers to expect the same endpoint on both Lichess and Lishogi.

I can't imagine there being many other consumers of this endpoint (like mobile applications) since for Lishogi this endpoint wasn't documented/announced yet.

[WandererXII]

LGTM, thanks!