[Dominators] Make applyUpdate's documentation less confusing [NFC]

Summary:
It was pointed out by @chandlerc that it's not clear whether both applyUpdates and insert/deleteEdge can be used to perform multiple updates.

IMO, the confusing part was that the comment above applyUpdates made a comparison of expected update time between calling it and calling insert/deleteEdge multiple times. It's generally not possible to safely call insert/deleteEdge multiple times, which documentation for each of the 3 functions warns about, so the whole comparison makes very little sense. On top of that, the comment is already lengthy, so I think it's best to just get rid of this comparison.

Reviewers: chandlerc, asbirlea, NutshellySima, grosser

Reviewed By: chandlerc

Subscribers: llvm-commits, chandlerc

Differential Revision: https://reviews.llvm.org/D49944

llvm-svn: 338184

llvm/include/llvm/Support/GenericDomTree.h

@@ -530,11 +530,10 @@ protected:
   /// CFG about its children and inverse children. This implies that deletions
   /// of CFG edges must not delete the CFG nodes before calling this function.
   ///
-  /// Batch updates should be generally faster when performing longer sequences
-  /// of updates than calling insertEdge/deleteEdge manually multiple times, as
-  /// it can reorder the updates and remove redundant ones internally.
-  /// The batch updater is also able to detect sequences of zero and exactly one
-  /// update -- it's optimized to do less work in these cases.
+  /// The applyUpdates function can reorder the updates and remove redundant
+  /// ones internally. The batch updater is also able to detect sequences of
+  /// zero and exactly one update -- it's optimized to do less work in these
+  /// cases.
   ///
   /// Note that for postdominators it automatically takes care of applying
   /// updates on reverse edges internally (so there's no need to swap the