document the reason for requiring force_ndarray_like (#59)

* document the reason for requiring force_ndarray_like

* update the changelog entry for a previous PR

* add a entry to whats-new.rst [skip-ci]

Author: keewis

docs/creation.rst

@@ -111,6 +111,9 @@ or overwrite the default registry:
     To properly work with ``xarray``, the ``force_ndarray_like`` or
     ``force_ndarray`` options have to be enabled on the custom registry.
 
+   Without it, python scalars wrapped by :py:class:`pint.Quantity` may raise errors or
+   have their units stripped.
+
 Saving with units
 -----------------
 In order to not lose the units when saving to disk, we first have to call the

docs/whats-new.rst

@@ -5,7 +5,10 @@ What's new
 ------------------
 - rewrite :py:meth:`Dataset.pint.quantify` and :py:meth:`DataArray.pint.quantify`,
   to use pint's `parse_units` instead of `parse_expression` (:pull:`40`)
-- refactor the internal conversion functions (:pull:``)
+- refactor the internal conversion functions (:pull:`56`)
+- allow converting indexes (except :py:class:`pandas.MultiIndex`) (:pull:`56`)
+- document the reason for requiring the ``force_ndarray_like`` or ``force_ndarray``
+  options on unit registries (:pull:`59`)
 
 v0.1 (October 26 2020)
 ----------------------