[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

llvm/include/llvm/Support/WithColor.h

@@ -110,7 +110,12 @@ public:
   /// outputting colored text, or before program exit.
   WithColor &resetColor();
 
+  /// Implement default handling for Error.
+  /// Print "error: " to stderr.
   static void defaultErrorHandler(Error Err);
+
+  /// Implement default handling for Warning.
+  /// Print "warning: " to stderr.
   static void defaultWarningHandler(Error Warning);
 };