[LangRef] update text for shufflevector

D72467 updated the shufflevector instruction to include a constant mask
rather than a mask operand. The LangRef text was vague enough to still
make sense, but it is better to update here too, so there's no confusion
about valid mask values. The text here is adapted from the documentation
code comments for "class ShuffleVectorInst".

Differential Revision: https://reviews.llvm.org/D77396