Document that filter doesn't override fetch-depth

[jakebailey]

As noted in #1396 (comment), if you want to make full use of filter: blob:none, you still need to set fetch-depth: 0, as the default is still 1 which means you still don't get any history.

Happy to change this to whatever makes sense.

[jenseng]

Or perhaps if a filter is set, fetch-depth could then implicitly default to 0? I can't think of a scenario where you'd want a simultaneously shallow AND partial clone.

edit: fixed 1 -> 0 😆

[jakebailey]

I assume you mean "default to 0", but yeah, that was something I thought about too, I just wasn't sure how much of a breaking change was palatable.