Document store CLI argument

[tomberek]

Fixes #5171 and #1556

Mostly took verbiage from previous issues and release notes.

[thufschmitt]

There’s a nix describe-stores command that generates a documentation for all the existing stores and their options. It would be great to use it to generate this documentation (or possibly an appendix as it’s more exhaustive but also less reader-friendly)

[Ericson2314]

Yeah we should generate that appendix and basically just point to it.

[Ericson2314]

I think we can close this now that we have nix help-stores?

[fricklerhandwerk]

Yes!