warn about .quantify setting the units of dimension coordinates as attributes (#126)

* make the note about dim coords in the docstring of quantify a warning

* update whats-new.rst [skip-ci]

* fill in the issue / PR numbers

Author: keewis

docs/whats-new.rst

@@ -8,6 +8,9 @@ What's new
 - allow special "no unit" values in :py:meth:`Dataset.pint.quantify` and
   :py:meth:`DataArray.pint.quantify` (:pull:`125`)
   By `Justus Magin <https://github.com/keewis>`_.
+- convert the note about dimension coordinates saving their units in the attributes a
+  warning (:issue:`124`, :pull:`126`)
+  By `Justus Magin <https://github.com/keewis>`_.
 
 0.2 (May 10 2021)
 -----------------

pint_xarray/accessors.py

@@ -260,6 +260,8 @@ def quantify(self, units=_default, unit_registry=None, **unit_kwargs):
             the data into memory. To avoid that, consider converting
             to ``dask`` first (e.g. using ``chunk``).
 
+        .. warning::
+
             As units in dimension coordinates are not supported until
             ``xarray`` changes the way it implements indexes, these
             units will be set as attributes.
@@ -915,6 +917,8 @@ def quantify(self, units=_default, unit_registry=None, **unit_kwargs):
             the data into memory. To avoid that, consider converting
             to ``dask`` first (e.g. using ``chunk``).
 
+        .. warning::
+
             As units in dimension coordinates are not supported until
             ``xarray`` changes the way it implements indexes, these
             units will be set as attributes.