Document pwsh shell completion

[heaths]

Based on discussion in #19408, update the documentation to specify that :pwsh must be passed explicitly and that it will translate to a "powershell" argument, as is currently supported by Go's common github.com/spf13/cobra module and Rust's common clap (with clap_complete) crate.

[heaths]

@MikeMcQuaid alternatively, what if shells accepted a T::Array[T.any(Symbol, Hash)] instead, such that devs could pass just :pwsh - matching the name of the executable but passing "powershell" as the completion arg, since that's what known completion handlers support; or, they could pass something like :pwsh => "pwsh" (or any string) to use a different name? Alternatively, we keep it simple and treat :pwsh as "pwsh" as an argument, and they could pass :pwsh => "powershell" if they want to pass a different argument. But, seems like extra work for what has so far been the only scenario. Again, though, it does keep it simple.

Maybe I'm just jumping the gun here, but figured may as well take care of the possibility now. Again, I haven't seen any completion arg that uses anything other than "powershell", but my experience is limited to a small handful of completion handlers.

That said, I still think it makes sense the completion dir uses "pwsh" since that's the name of the executable, just like "bash" is, well, "bash" instead of "bourne" or something.

[MikeMcQuaid]

@heaths I think it's fine as-is for now.

[MikeMcQuaid]

Thanks! Seems enough for now.