text: Title: DOC: pandas.DataFrame.aggregate return value
  Body: ### Pandas version checks
  
  - [X] I have checked that the issue still exists on the latest versions of the docs on `main` [here](https://pandas.pydata.org/docs/dev/)
  
  
  ### Location of the documentation
  
  https://pandas.pydata.org/docs/dev/reference/api/pandas.DataFrame.aggregate.html#pandas.DataFrame.aggregate
  
  ### Documentation problem
  
  The documentation of pandas.DataFrame.aggregate() method says:
  
      The return can be:
  
      * scalar : when Series.agg is called with single function
      * Series : when DataFrame.agg is called with a single function
      * DataFrame : when DataFrame.agg is called with several functions
   
  But 
  df = pd.DataFrame([[1]]) ; type(df.agg(lambda x: 3*x))
  returns pandas.core.frame.DataFrame even though .agg() was called with a single function
  
  ### Suggested fix for documentation
  
  I'd love to offer a fix, but the reason I was looking up the docs was that I'd like to know what .agg() does exactly...