gh-80678: Document the preferred delimiters of csv.Sniffer

Doc/library/csv.rst

@@ -322,6 +322,12 @@ The :mod:`!csv` module defines the following classes:
       in particular if it is a single column,
       so there is no delimiter to find.
 
+      If several combinations fit the sample equally well ---
+      for example if both ``','`` and ``';'`` split every row consistently ---
+      the delimiters ``','``, ``'\t'``, ``';'``, ``' '`` and ``':'``
+      are preferred, in this order,
+      no matter how many times each of them occurs.
+
       .. versionchanged:: next
          The dialect is now deduced by trial parsing
          and the results may differ from those of earlier Python versions.

Lib/csv.py

@@ -260,6 +260,10 @@ def sniff(self, sample, delimiters=None):
         beginning, then, after eliminating the combinations which are
         clearly worse than the leader, a several times larger part,
         and so on.
+
+        If several combinations fit the sample equally well, the
+        delimiters listed in the preferred attribute are preferred, in
+        that order, no matter how many times each of them occurs.
         """
         import re
         from collections import defaultdict