Document the config merge strategy

[HarHarLinks]

matrix-authentication-service/docs/setup/general.md

Line 50 in d3eb5be

To help understand what the resulting configuration looks like after merging all the configuration files, the [`config dump`](../reference/cli/config.md#config-dump) command can be used:

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?

[sandhose]

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

[HarHarLinks]

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.

[matrixbot]

For your information, this issue has been copied over to the Element fork of matrix-authentication-service: element-hq/matrix-authentication-service#3136