FR: add fsmonitor docs

[arxanas]

There should probably be a document under https://github.com/martinvonz/jj/tree/main/docs explaining how to install, configure, and troubleshoot a filesystem monitor. See #1841 (comment), #1841 (comment), #1841 (comment) for some relevant items.

[martinvonz]

It's now enabled by default, and we have docs describing the other setup, so I think we can close this. We might need to remind packagers to enable the feature. I don't know how good they generally are at noticing what default features change in Rust projects.