DOC: Add notes about M and Y to to_timedelata documentation. (pandas-…

…dev#34968) (pandas-dev#34979)

* DOC: Add notes about M and Y to to_timedelata documentation.  (pandas-dev#34968)

* DOC: Update notes about M and Y to to_timedelata documentation.  (pandas-dev#34968)

* DOC: Add notes about M and Y to to_timedelata documentation.  (pandas-dev#34968)

pandas/core/tools/timedeltas.py

@@ -25,7 +25,10 @@ def to_timedelta(arg, unit=None, errors="raise"):
     Parameters
     ----------
     arg : str, timedelta, list-like or Series
-        The data to be converted to timedelta.
+        The data to be converted to timedelta.  The character M by itself,
+        e.g. '1M', is treated as minute, not month. The characters Y and y
+        are treated as the mean length of the Gregorian calendar year -
+        365.2425 days or 365 days 5 hours 49 minutes 12 seconds.
     unit : str, optional
         Denotes the unit of the arg for numeric `arg`. Defaults to ``"ns"``.