[Debuginfo][NFC] add comments for WithColor routines.

Summary:
This patch is follow-up for D74481. It adds comments
to WithColor::defaultErrorHandler() and
WithColor::defaultWarningHandler().

Reviewers: jhenderson, dblaikie, JDevlieghere

Reviewed By: JDevlieghere

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D74742