Document options available to pass as "URL parameters" to binary cache URL:s

[terlar]

Is your feature request related to a problem? Please describe.
There are a bunch of useful parameters that you can pass to the binary cache URL:s in order to affect the Nix command.

For example (compression and secret-key...):
https://github.com/NixOS/nix/blob/master/src/libstore/binary-cache-store.hh#L19-L30

Describe the solution you'd like
I would like some documentation for the URL parameters available to be given to binary cache stores.

Similar to how it has been documented for S3 substituter.

Describe alternatives you've considered
Reading the source code and tests provided for nix.

Additional context
Add any other context or screenshots about the feature request here.

[thufschmitt]

See #5467. It never made it to the documentation, but there’s a nix describe-stores commands that generates a markdown description of all the stores and their options.