[VectorOps] Update vector transfer read op comments.

Summary: Update vector transfer read op comments.

Reviewers: nicolasvasilache, aartbik

Reviewed By: nicolasvasilache, aartbik

Subscribers: merge_guards_bot, mehdi_amini, rriddle, jpienaar, burmako, shauheen, antiagainst, arpith-jacob, mgester, lucyrfox, liufengdb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72731