reference to iso8601 standard

Author: minggli

pandas/core/tools/datetimes.py

@@ -250,8 +250,9 @@ def to_datetime(arg, errors='raise', dayfirst=False, yearfirst=False,
     >>> %timeit pd.to_datetime(s,infer_datetime_format=False)
     1 loop, best of 3: 471 ms per loop
 
-    Passing require_iso8601=True will only parse ISO8601-compliant datetime
-    strings and treat others as non-parseable dates.
+    Passing require_iso8601=True will only parse datetime strings compliant
+    to `ISO8601<https://en.wikipedia.org/wiki/ISO_8601>`_ and treat others
+    as non-parseable dates.
 
     >>> s = pd.Series(['M1809', 'M1701', pd.Timestamp('20130101')])