Update where docstring to make return value type more clear (#3408)

* Update where docstring to make return value type more clear

The where docstring was a little confusing to me.  I misunderstood "Same type as caller' to mean the values in the xarray not the xarray itself.  I think this small change will clean it up for most users.  Thanks.

* Update xarray/core/common.py

Co-Authored-By: Maximilian Roos <5635139+max-sixty@users.noreply.github.com>

xarray/core/common.py

@@ -1069,7 +1069,7 @@ def where(self, cond, other=dtypes.NA, drop: bool = False):
 
         Returns
         -------
-        Same type as caller.
+        Same xarray type as caller, with dtype float64.
 
         Examples
         --------