[mlir][vector] Clarify OOB semantics for `gather` and `scatter`

Reword the vector gather and scatter op description to make it
well-defined to have out-of-bounds indices when the corresponding mask
bits are false.

Update the code sample to avoid relying on C UB semantics to provide
informal semantics for vector.gather.

This change should be consistent with the existing interpretation of the
semantics in the codebase.

Issue: https://github.com/llvm/llvm-project/issues/60905

Reviewed By: dcaballe

Differential Revision: https://reviews.llvm.org/D145824