docs: document stable sortby ordering (#11491)

* docs: document stable sortby ordering

* docs: document stable sortby ordering

Author: Whning0513

xarray/core/dataarray.py

@@ -5234,6 +5234,8 @@ def sortby(
         https://numpy.org/doc/stable/reference/generated/numpy.lexsort.html
         and the FIRST key in the sequence is used as the primary sort key,
         followed by the 2nd key, etc.
+        Sorting is stable: when all sort keys compare equal, the original order is
+        preserved.
 
         Parameters
         ----------

xarray/core/dataset.py

@@ -8124,6 +8124,8 @@ def sortby(
         https://numpy.org/doc/stable/reference/generated/numpy.lexsort.html
         and the FIRST key in the sequence is used as the primary sort key,
         followed by the 2nd key, etc.
+        Sorting is stable: when all sort keys compare equal, the original order is
+        preserved.
 
         Parameters
         ----------