Clarify relation between DateOffset and relativedelta (#42251)

dateutil.relativedelta can be constructed either from two date/datetime objects passed as positional arguments, or explicitly constructed in terms of year, month, day etc passed as keyword arguments. DateOffset only supports the keyword argument form of relativedelta.

Author: daviewales

pandas/_libs/tslibs/offsets.pyx

@@ -1152,12 +1152,13 @@ class DateOffset(RelativeDeltaOffset, metaclass=OffsetMeta):
     """
     Standard kind of date increment used for a date range.
 
-    Works exactly like relativedelta in terms of the keyword args you
-    pass in, use of the keyword n is discouraged-- you would be better
+    Works exactly like the keyword argument form of relativedelta.
+    Note that the positional argument form of relativedelata is not
+    supported. Use of the keyword n is discouraged-- you would be better
     off specifying n in the keywords you use, but regardless it is
     there for you. n is needed for DateOffset subclasses.
 
-    DateOffset work as follows.  Each offset specify a set of dates
+    DateOffset works as follows.  Each offset specify a set of dates
     that conform to the DateOffset.  For example, Bday defines this
     set to be the set of dates that are weekdays (M-F).  To test if a
     date is in the set of a DateOffset dateOffset we can use the