Adding to the function docs that groupby.transform() function parameter can be a string (#50002)

Author: seanjedi

pandas/core/groupby/groupby.py

@@ -399,7 +399,7 @@ class providing the base-class of operations.
 
 Parameters
 ----------
-f : function
+f : function, str
     Function to apply to each group. See the Notes section below for requirements.
 
     Can also accept a Numba JIT function with