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

[kuhar]

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.

[chandlerc]

LGTM! Thanks for the improved comment. Hopefully at least helps future me when I get confused reading it again.

llvm/trunk/include/llvm/Support/GenericDomTree.h

#endif
   ///    logically it results in a single insertions.
   ///  - {{Insert, A, B}, {Insert, A, B}} is invalid, because it doesn't make
   ///    sense to insert the same edge twice.
   ///
   /// What's more, the functions assumes that it's safe to ask every node in the
   /// 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
   /// From and To pointers when constructing DominatorTree::UpdateType).
   /// The type of updates is the same for DomTreeBase<T> and PostDomTreeBase<T>
   /// with the same template parameter T.
   ///
   /// \param Updates An unordered sequence of updates to perform.