xtask: fix interplay between fmt-on-save and xsync

[daprilik]

Fixing the underlying issue from #914

This PR does a few things:

1. The xtask-path logic has been fixed to always drop the file under target/xtask-path, regardless of whether a --custom-root is being used or not.
2. A new --run-on-save parameter has been added to xtask, which can be used to skip certain expensive checks when running xtask in response to a file save event. It is not currently being used by the open-source xtask, but plays an important role in the internal repo's xtask fmt implementation.
3. Updates the Guide to make it more clear why one may or may not want to opt into this functionality.

These changes are not particularly interesting in isolation, but make more sense when reviewed alongside the changes being made in the internal repo.

The fact we have this sort of coupling between the oss xtask and the closed-source xtask is unfortunate, and something we should try and move away from in the future...

[jstarks]

Thanks for the fixes and particularly for the guide clarifications.