Backport PR pandas-dev#52596: DOC Added that DatetimeIndex needs to be ordered on first()

Author: DeaMariaLeon

pandas/core/generic.py

@@ -8905,7 +8905,7 @@ def first(self: NDFrameT, offset) -> NDFrameT:
         """
         Select initial periods of time series data based on a date offset.
 
-        When having a DataFrame with dates as index, this function can
+        For a DataFrame with a sorted DatetimeIndex, this function can
         select the first few rows based on a date offset.
 
         Parameters