Clarify `limit` parameter on `/context`

Nobody has claimed this yet.

**Link to problem area**: https://spec.matrix.org/v1.15/client-server-api/#get_matrixclientv3roomsroomidcontexteventid

**Issue**

> limit integer
> The maximum number of context events to return. The limit applies to the sum of the events_before and events_after arrays. The requested event ID is always returned in event even if limit is 0. Defaults to 10.

The limit is specified as divided between `events_before` and `events_after`, which for odd limits implies a remainder. Rust-SDK tests that this remainder is given to `events_after`, despite a naive implementation discarding it with two truncated integer divisions. It probably derived this idea from some other implementation, establishing a de facto specification which should be reflected here.

Start with the linked Matrix Client-Server API section for GET /rooms/{roomId}/context/{eventId}, then compare its limit wording with the Rust-SDK behavior described in the issue. Clarify how an odd limit is divided between events_before and events_after, including where the remainder goes; done means the specification removes this ambiguity.

Written by the indexing model from the issue text.