DOC: Fix versionadded for cond in .where (#15509)

[ci skip]

Author: gfyoung

pandas/core/generic.py

@@ -4910,13 +4910,15 @@ def _where(self, cond, other=np.nan, inplace=False, axis=None, level=None,
             not change input %(klass)s (though pandas doesn't check it).
 
             .. versionadded:: 0.18.1
+                A callable can be used as cond.
 
         other : scalar, %(klass)s, or callable
             If other is callable, it is computed on the %(klass)s and
             should return scalar or %(klass)s. The callable must not
             change input %(klass)s (though pandas doesn't check it).
 
             .. versionadded:: 0.18.1
+                A callable can be used as other.
 
         inplace : boolean, default False
             Whether to perform the operation in place on the data