Document the config merge strategy

[matrixbot]

This issue was originally created by @HarHarLinks at matrix-org/matrix-authentication-service#3136.

https://github.com/matrix-org/matrix-authentication-service/blob/d3eb5be63ead3078f71f142785fc3768dcd7bcd8/docs/setup/general.md?plain=1#L50

but it's not explained how configs get merged in theory. will values from second.yaml overwrite those in first.yaml or reverse? or is it illegal to try this?

[matrixbot]

This comment was originally posted by @sandhose at matrix-org/matrix-authentication-service#3136 (comment).

We're using figment under the hood, with the merge strategy, meaning that later values are generally preferred. Not sure whether this is the right approach or not; it may make more sense to switch to the adjoin strategy, where the first config files take precedence, and arrays get merged?

https://docs.rs/figment/latest/figment/struct.Figment.html#conflict-resolution

[matrixbot]

This comment was originally posted by @HarHarLinks at matrix-org/matrix-authentication-service#3136 (comment).

merge strategy meets what I would have guessed but could not be sure. Whether you decide to change it or not, this issue is about documenting your approach so that the reader may create their config appropriately.