add polyval to polyfit see also (#5020)

* add polyval to polyfit see also

* whatsnew

* link polyval to docs

* link polyval to docs

Author: aaronspring

doc/whats-new.rst

@@ -55,6 +55,9 @@ Documentation
 ~~~~~~~~~~~~~
 - New section on :ref:`add_a_backend` in the "Internals" chapter aimed to backend developers
   (:issue:`4803`, :pull:`4810`). By `Aureliana Barghini <https://github.com/aurghs>`_.
+- Add :py:meth:`Dataset.polyfit` and :py:meth:`DataArray.polyfit` under "See also" in
+  the docstrings of :py:meth:`Dataset.polyfit` and :py:meth:`DataArray.polyfit`
+  (:issue:`5016`, :pull:`5020`). By `Aaron Spring <https://github.com/aaronspring>`_.
 
 
 Internal Changes

xarray/core/dataarray.py

@@ -3781,6 +3781,8 @@ def polyfit(
         See Also
         --------
         numpy.polyfit
+        numpy.polyval
+        xarray.polyval
         """
         return self._to_temp_dataset().polyfit(
             dim, deg, skipna=skipna, rcond=rcond, w=w, full=full, cov=cov

xarray/core/dataset.py

@@ -6399,6 +6399,8 @@ def polyfit(
         See Also
         --------
         numpy.polyfit
+        numpy.polyval
+        xarray.polyval
         """
         variables = {}
         skipna_da = skipna