doc: Add what's new entry for __repr__ changes

Sorted as a "breaking change", for 0.19.1 for now.

Author: st-bender

doc/whats-new.rst

@@ -27,6 +27,11 @@ New Features
 Breaking changes
 ~~~~~~~~~~~~~~~~
 
+- The ``__repr__`` of a :py:class:`xarray.Dataset`'s ``attrs``, ``coords``,
+  and ``data_vars`` ignore ``xarray.set_option(display_max_rows=...)`` and
+  show the full output when called directly as, e.g., ``ds.data_vars`` or
+  ``print(ds.data_vars)`` (:issue:`5545`, :pull:`5580`).
+  By `Stefan Bender <https://github.com/st-bender>`_.
 
 Deprecations
 ~~~~~~~~~~~~