Improve Hash#values_at docs

[j8r]

Closes #9952.

[docelic]

Hey @j8r, I was looking at the definition of methods #key_index and #values_at.

def key_index(key) 
def values_at(*indexes : K)

Both of these functions accept the key(s) as argument, not indexes.
In that context, I believe that, while you are at it, values_at should actually be fixed to say *keys rather than *indexes, right?

[straight-shoota]

@docelic That's already in this PR!

[j8r]

That's not a breaking change, how can it be @straight-shoota ?

[straight-shoota]

I probably didn't realize the argument rename doesn't have a practical relevance because it's a splat.