Improvements to user documentation

[BinaryWarlock]

I'm a new user and found the user documentation rather sparse and had me googling a lot to figure out what is supported and how to do it.

• The docs don't show how to enable compression. The only mention of it is in the FAQ (which just vaguely references a --compression flag, but really you need to initialize it with rustic init --set-compression <N>, which is difficult to find.)
  This should be mentioned in the Getting Started guide and Initializing a Repository. It should also be mentioned that the default is no compression.

• There's no message indicating that compression was enabled, nor is it clear how to check if it's enabled on a repository. I believe rustic init should indicate the compression level and it should show up in the config or repoinfo somewhere.

• It's not clear if there is a way to limit the number of threads used by rustic backup. It seems to use a lot of CPU time for the duration of it (which may be hours), which can slow down other tasks, so I'd rather the level of parallelism be configurable.

• It's not clear how to verify if a set of local files (or a local directory) is present and validate it in snapshots. Googling shows there is a rustic diff that may do this, but that should be documented feature.

• Related: Is there something like borgmatic's spot check, i.e., giving it a local directory and telling it to verify x% of it with a snapshot? If not, that would be awesome. I know that rustic check has --read-data-subset but it cannot, AFAICT, take in a local directory as a source.

• --no-cache / no-cache = true should probably be mentioned in the Getting Started guide and "Preparing a new repository > Local backend" for local backends.

• It's not stated what encryption is used.

[aawsome]

Hi @BinaryWarlock - thanks a lot for opening this issue.

We do know that our docu can be improved, this is just a matter of time as good docu is hard to make. We however love to see contributions on the docu - even small incremental improvements help a lot. You can either open a pull-request via github or use the button "suggest an edit" on the top right of https://rustic.cli.rs/docs/. Of course, please don't hesitate to ask if you want to help but run into problems when doing so!

About some of your points:

• compression: The default is enabled compression. You have to use --set-compression to explicitly disable it
• Showing compression info or limit number of threads are feature requests to rustic. The latter is already addressed, can you please open one for the compression show topic?
• Same applies to the "spot check" feature, which IMO would be a feature to diff, which I personally find a very good idea to add!
• used encryption is mentioned in the linked "restic repository format"