Include guidelines how to backfill with `/messages` and `/context`

[MadLittleMods]

Link to problem area:

https://spec.matrix.org/v1.1/server-server-api/#backfilling-and-retrieving-missing-events

Issue

As brought up in matrix-org/complement#492 (review) and matrix-org/matrix-spec-proposals#3030 (comment), there is no right answer for when and how to backfill events.

The spec just says this:

Since users in that room are able to request the history by the /messages client API endpoint, it’s possible that they might step backwards far enough into history before the homeserver itself was a member of that room.

To cover this case, the federation API provides a server-to-server analog of the /messages client API, allowing one homeserver to fetch history from another. This is the /backfill API.

-- https://spec.matrix.org/v1.1/server-server-api/#backfilling-and-retrieving-missing-events

But it should probably also mention that /context should backfill at the very least. And have some heuristics for when exactly to backfill. Is it acceptable to have gaps (MSC3871)? etc