Add cache location hint to persist option documentation

[maweigert]

Small change in the doc of the persist option (the earlier given cache location is different on OSX for example)

[tlambert03]

thanks again. Since this (and pretty much all of the options on @magicgui) are just parameters for magicgui.widgets.FunctionGui. We should also add it to the docs there too. Also, I don't love adding a private module.parameter to the docstring like that, so maybe we should make _util.py public? Alternatively, fyi, you can get the specific cache path of each generated widget at widget._dump_path. Not sure if you want to mention that... we could make that public (and possible change the name to cache_path). Thoughts?

[maweigert]

Sure, maybe even add that cache location may vary is already enough (instead of changing visibility of a whole module). I was just a bit tripped up by the current doc and that might be the case for others to.

[tlambert03]

I was just a bit tripped up by the current doc and that might be the case for others to.

yeah, def... that was outdated. much appreciated. OK, let's just use this as is for now and I may extend it later. thanks again!