Document atomic() deferred BEGIN, savepoint behavior and connection modes

Documents that atomic() opens a deferred BEGIN, that calling it
while the connection is already inside a transaction produces a
savepoint whose changes only persist when the outer transaction
commits, and that Python 3.12+ autocommit=True/False connections
are not supported. Adds a test pinning the savepoint-inside-manual-
BEGIN behavior.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UnLnhsH25Nnv7LHhekUfPd

tests/test_atomic.py

@@ -172,3 +172,20 @@ def test_transform_detects_foreign_key_check_violations(fresh_db):
 
     assert fresh_db["books"].foreign_keys == []
     assert fresh_db.conn.execute("PRAGMA foreign_keys").fetchone()[0]
+
+
+def test_atomic_inside_manual_transaction_uses_savepoint(fresh_db):
+    fresh_db["t"].insert({"id": 1}, pk="id")
+    fresh_db.execute("begin")
+    with fresh_db.atomic():
+        fresh_db["t"].insert({"id": 2}, pk="id")
+    # Nothing is committed until the user's own transaction commits
+    assert fresh_db.conn.in_transaction
+    fresh_db.conn.rollback()
+    assert [r["id"] for r in fresh_db["t"].rows] == [1]
+    # And with a commit instead, the atomic block's writes persist
+    fresh_db.execute("begin")
+    with fresh_db.atomic():
+        fresh_db["t"].insert({"id": 3}, pk="id")
+    fresh_db.conn.commit()
+    assert [r["id"] for r in fresh_db["t"].rows] == [1, 3]