Documentation: Remove an obsolete note about the implementation of `Cord`.

PiperOrigin-RevId: 477587157
Change-Id: I4527b891222a011f8c1788387479bdc8e4a3bb94

absl/strings/cord.h

@@ -20,8 +20,7 @@
 // structure. A Cord is a string-like sequence of characters optimized for
 // specific use cases. Unlike a `std::string`, which stores an array of
 // contiguous characters, Cord data is stored in a structure consisting of
-// separate, reference-counted "chunks." (Currently, this implementation is a
-// tree structure, though that implementation may change.)
+// separate, reference-counted "chunks."
 //
 // Because a Cord consists of these chunks, data can be added to or removed from
 // a Cord during its lifetime. Chunks may also be shared between Cords. Unlike a