normalize and document __fish_status_to_signal

[IlanCosman]

I think that this is a case exactly like prompt_pwd, where Fish can offer a convenient service for those creating a prompt. I currently use this undocumented function in my prompt, Tide (well technically i'm using __fish_pipestatus_with_signal because this is before 293a3a6 is released).

[IlanCosman]

Does the fish-future tag mean that the idea is accepted @faho? I'm happy to open a PR if that is the case.

[faho]

Does the fish-future tag mean that the idea is accepted @faho? I'm happy to open a PR if that is the case.

"fish-future" means that it's something we'll add in "the future" - whenever that is. So it is accepted, yeah.

In contrast "fish-X.Y" means we'd like to work on it in that cycle, if possible, and "next-major" means it'll have to wait for the next major release - whenever that is. I didn't tag it as fish-3.2.0 because I don't think this should hold up the 3.2.0 release at this point.