gh-80678: Document the preferred attribute of csv.Sniffer (GH-155068)

It can be modified to change the order in which the delimiters are
preferred for breaking ties.
(cherry picked from commit 1370f8a)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

Author: serhiy-storchaka

Doc/library/csv.rst

@@ -316,8 +316,8 @@ The :mod:`!csv` module defines the following classes:
 
       If several delimiters fit the sample equally well ---
       for example if both ``','`` and ``';'`` split every row consistently ---
-      the delimiters ``','``, ``'\t'``, ``';'``, ``' '`` and ``':'``
-      are preferred, in this order,
+      the delimiters listed in the :attr:`~Sniffer.preferred` attribute
+      are preferred, in that order,
       no matter how many times each of them occurs.
 
    .. method:: has_header(sample)
@@ -339,6 +339,15 @@ The :mod:`!csv` module defines the following classes:
       This method is a rough heuristic and may produce both false positives and
       negatives.
 
+   The :class:`Sniffer` class has the following attribute:
+
+   .. attribute:: preferred
+
+      The list of the delimiters preferred for breaking ties,
+      in the order of preference.
+      It can be modified.
+      Its initial value is ``[',', '\t', ';', ' ', ':']``.
+
 An example for :class:`Sniffer` use::
 
    with open('example.csv', newline='') as csvfile: