[DTU] Refine the document of mutation APIs [NFC] (PR40528)

Summary:
It was pointed out in [[ https://bugs.llvm.org/show_bug.cgi?id=40528 | Bug 40528 ]] that it is not clear whether insert/deleteEdge can be used to perform multiple updates and [[ https://reviews.llvm.org/D57316#1388344 | a comment in D57316 ]] reveals that the difference between several ways to update the DominatorTree is confusing.

This patch tries to address issues above.

Reviewers: mkazantsev, kuhar, asbirlea, chandlerc, brzycki

Reviewed By: mkazantsev, kuhar, brzycki

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D57881

llvm-svn: 354437