Clarify `[T]::select_nth_unstable*` return values

In cases where the nth element is not unique within the slice, it is not
correct to say that the values in the returned triplet include ones for
"all elements" less/greater than that at the given index: indeed one (or
more) such values would then laso contain values equal to that at the
given index.

The text proposed here clarifies exactly what is returned, but in so
doing it is also documenting an implementation detail that previously
wasn't detailed: namely that the return slices are slices into the
reordered slice.  I don't think this can be contentious, because the
lifetimes of those returned slices are bound to that of the original
(now reordered) slice—so there really isn't any other reasonable
implementation that could have this behaviour; but nevertheless it's
probably best if @rust-lang/libs-api give it a nod?

Fixes #97982
r? m-ou-se

@rustbot label +A-docs C-bug +T-libs-api

0 files changed, 0 insertions, 0 deletions