[LangRef][VP] Document vp.gather and vp.scatter intrinsics

This patch fleshes out the missing documentation for the final two VP
intrinsics introduced in D99355: `llvm.vp.gather` and `llvm.vp.scatter`.
It does so mostly by deferring to the `llvm.masked.gather` and
`llvm.masked.scatter` intrinsics, respectively.

Reviewed By: simoll

Differential Revision: https://reviews.llvm.org/D112997