Add a comment about a more efficient implementation of btree range erase.

PiperOrigin-RevId: 561954737
Change-Id: I0e29a4f4523e4b3eafbd26d1d96f6e1c8deed957

absl/container/internal/btree.h

diff --git a/absl/container/internal/btree.h b/absl/container/internal/btree.h
index 569faa0..5e8a252 100644
--- a/absl/container/internal/btree.h
+++ b/absl/container/internal/btree.h
@@ -2534,6 +2534,10 @@
   return res;
 }
 
+// Note: we tried implementing this more efficiently by erasing all of the
+// elements in [begin, end) at once and then doing rebalancing once at the end
+// (rather than interleaving deletion and rebalancing), but that adds a lot of
+// complexity, which seems to outweigh the performance win.
 template <typename P>
 auto btree<P>::erase_range(iterator begin, iterator end)
     -> std::pair<size_type, iterator> {