gh-61105: Add default param, note on using cookiejar subclass (GH-95427)

(cherry picked from commit 5eaf4d6101)

Co-authored-by: Stanley <46876382+slateny@users.noreply.github.com>

Doc/library/http.cookiejar.rst

@@ -61,7 +61,7 @@ The following classes are provided:
    responsible for storing and retrieving cookies from a file or database.
 
 
-.. class:: FileCookieJar(filename, delayload=None, policy=None)
+.. class:: FileCookieJar(filename=None, delayload=None, policy=None)
 
    *policy* is an object implementing the :class:`CookiePolicy` interface.  For the
    other arguments, see the documentation for the corresponding attributes.
@@ -71,6 +71,8 @@ The following classes are provided:
    :meth:`load` or :meth:`revert` method is called.  Subclasses of this class are
    documented in section :ref:`file-cookie-jar-classes`.
 
+   This should not be initialized directly – use its subclasses below instead.
+
    .. versionchanged:: 3.8
 
       The filename parameter supports a :term:`path-like object`.
@@ -317,7 +319,7 @@ FileCookieJar subclasses and co-operation with web browsers
 The following :class:`CookieJar` subclasses are provided for reading and
 writing.
 
-.. class:: MozillaCookieJar(filename, delayload=None, policy=None)
+.. class:: MozillaCookieJar(filename=None, delayload=None, policy=None)
 
    A :class:`FileCookieJar` that can load from and save cookies to disk in the
    Mozilla ``cookies.txt`` file format (which is also used by curl and the Lynx
@@ -338,7 +340,7 @@ writing.
    Mozilla.
 
 
-.. class:: LWPCookieJar(filename, delayload=None, policy=None)
+.. class:: LWPCookieJar(filename=None, delayload=None, policy=None)
 
    A :class:`FileCookieJar` that can load from and save cookies to disk in format
    compatible with the libwww-perl library's ``Set-Cookie3`` file format.  This is