FR: `jj squash` without args can be better documented

[ffyuanda]

Is your feature request related to a problem? Please describe.
When I first read the docs for jj squash, I needed to assume that running jj squash with no arguments implies -r @. jj squash with no argument is a very common workflow, but it is not explicitly documented, even though it's easy to guess/try.

Describe the solution you'd like
We should document what happens when running jj squash without arguments. Something like "Without any options, moves the changes from the working-copy commit to the parent revision." or "Without any options, implies '-r @' to move the changes from the working-copy commit to the parent revision."

[ffyuanda]

I can open a PR for this.

[joyously]

coordinate with this one: #8235 Improve squash command description

[ffyuanda]

I think this is a separate issue from #7661 (which #8235 is solving). Since I'm new to the community, I want to leverage this opportunity to try the contribution process. Will open a PR on this, thanks!