gh-105116: Document that an escaped csv quotechar does not cause quoting (GH-155073)

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: serhiy-storchaka

Doc/library/csv.rst

@@ -386,6 +386,8 @@ The :mod:`!csv` module defines the following constants:
    Instructs :class:`writer` objects to only quote those fields which contain
    special characters such as *delimiter*, *quotechar*, ``'\r'``, ``'\n'``
    or any of the characters in *lineterminator*.
+   If *doublequote* is :const:`False` and *escapechar* is set,
+   the *quotechar* is escaped instead of causing the field to be quoted.
 
 
 .. data:: QUOTE_NONNUMERIC