DOC: complete parameter descriptions for DataFrame.insert's docstring (pandas-dev#55172)

complete parameter descriptions for DataFrame.insert's docstring

issue: jorisvandenbossche/pydata-amsterdam-pandas-sprint#1
- bullet point item pandas-dev#5: "complete the parameter descriptions"

Author: mcnoat

pandas/core/frame.py

@@ -4915,7 +4915,9 @@ def insert(
         column : str, number, or hashable object
             Label of the inserted column.
         value : Scalar, Series, or array-like
+            Content of the inserted column.
         allow_duplicates : bool, optional, default lib.no_default
+            Allow duplicate column labels to be created.
 
         See Also
         --------