Backport PR pandas-dev#52311: DOC: Clarify Timestamp(unit=) will not necessarily set unit

Author: mroeschke

pandas/_libs/tslibs/timestamps.pyx

@@ -1290,6 +1290,9 @@ class Timestamp(_Timestamp):
         Unit used for conversion if ts_input is of type int or float. The
         valid values are 'D', 'h', 'm', 's', 'ms', 'us', and 'ns'. For
         example, 's' means seconds and 'ms' means milliseconds.
+
+        For float inputs, the result will be stored in nanoseconds, and
+        the unit attribute will be set as ``'ns'``.
     fold : {0, 1}, default None, keyword-only
         Due to daylight saving time, one wall clock time can occur twice
         when shifting from summer to winter time; fold describes whether the