bpo-8145: Improve isolation_level documentation (GH-8499)

Initial patch by R. David Murray.

Doc/library/sqlite3.rst

@@ -281,7 +281,7 @@ Connection Objects
 
    .. attribute:: isolation_level
 
-      Get or set the current isolation level. :const:`None` for autocommit mode or
+      Get or set the current default isolation level. :const:`None` for autocommit mode or
       one of "DEFERRED", "IMMEDIATE" or "EXCLUSIVE". See section
       :ref:`sqlite3-controlling-transactions` for a more detailed explanation.
 
@@ -1010,22 +1010,30 @@ timestamp converter.
 Controlling Transactions
 ------------------------
 
-By default, the :mod:`sqlite3` module opens transactions implicitly before a
-Data Modification Language (DML)  statement (i.e.
-``INSERT``/``UPDATE``/``DELETE``/``REPLACE``).
-
-You can control which kind of ``BEGIN`` statements sqlite3 implicitly executes
-(or none at all) via the *isolation_level* parameter to the :func:`connect`
-call, or via the :attr:`isolation_level` property of connections.
+The underlying ``sqlite3`` library operates in ``autocommit`` mode by default,
+but the Python :mod:`sqlite3` module by default does not.
 
-If you want **autocommit mode**, then set :attr:`isolation_level` to ``None``.
+``autocommit`` mode means that statements that modify the database take effect
+immediately.  A ``BEGIN`` or ``SAVEPOINT`` statement disables ``autocommit``
+mode, and a ``COMMIT``, a ``ROLLBACK``, or a ``RELEASE`` that ends the
+outermost transaction, turns ``autocommit`` mode back on.
 
-Otherwise leave it at its default, which will result in a plain "BEGIN"
-statement, or set it to one of SQLite's supported isolation levels: "DEFERRED",
-"IMMEDIATE" or "EXCLUSIVE".
+The Python :mod:`sqlite3` module by default issues a ``BEGIN`` statement
+implicitly before a Data Modification Language (DML) statement (i.e.
+``INSERT``/``UPDATE``/``DELETE``/``REPLACE``).
 
-The current transaction state is exposed through the
-:attr:`Connection.in_transaction` attribute of the connection object.
+You can control which kind of ``BEGIN`` statements :mod:`sqlite3` implicitly
+executes via the *isolation_level* parameter to the :func:`connect`
+call, or via the :attr:`isolation_level` property of connections.
+If you specify no *isolation_level*, a plain ``BEGIN`` is used, which is
+equivalent to specifying ``DEFERRED``.  Other possible values are ``IMMEDIATE``
+and ``EXCLUSIVE``.
+
+You can disable the :mod:`sqlite3` module's implicit transaction management by
+setting :attr:`isolation_level` to ``None``.  This will leave the underlying
+``sqlite3`` library operating in ``autocommit`` mode.  You can then completely
+control the transaction state by explicitly issuing ``BEGIN``, ``ROLLBACK``,
+``SAVEPOINT``, and ``RELEASE`` statements in your code.
 
 .. versionchanged:: 3.6
    :mod:`sqlite3` used to implicitly commit an open transaction before DDL