Add epoch alternative to deprecation message (#21060)

Author: jorisvandenbossche

pandas/_libs/tslibs/timedeltas.pyx

@@ -1196,6 +1196,9 @@ class Timedelta(_Timedelta):
                 msg = textwrap.dedent("""\
                 Floor division between integer array and Timedelta is
                 deprecated. Use 'array // timedelta.value' instead.
+                If you want to obtain epochs from an array of timestamps,
+                you can rather use
+                'array - pd.Timestamp("1970-01-01")) // pd.Timedelta("1s")'.
                 """)
                 warnings.warn(msg, FutureWarning)
                 return other // self.value