[mlir] [VectorOps] Add missing comments to CreateMaskOp lowering

Summary: Add missing comment to CreateMask. Fixed typo in ConstantMask comment.

Reviewers: nicolasvasilache, rriddle, reidtatge, ftynse

Reviewed By: ftynse

Subscribers: mehdi_amini, rriddle, jpienaar, shauheen, antiagainst, nicolasvasilache, arpith-jacob, mgester, lucyrfox, liufengdb, stephenneuendorffer, Joonsoo, grosul1, frgossen, Kayjukh, jurahul

Tags: #mlir

Differential Revision: https://reviews.llvm.org/D81125