Docs: how to change a database-scope engine setting (e.g. the vector rebuild ratio) from Python #112

The docs never say how to change an engine setting of one database from Python, and the create_vector_index docstring (since #111) tells users to "lower rebuildGraphRatio (a database-scope setting)" without saying how. Every SCOPE.DATABASE setting can be set with SQL, which the bindings already run: db.command("sql", "ALTER DATABASE arcadedb.vectorIndex.rebuildGraphRatio 0.05"). Checked on engine main c3af6d9028 (JDK 25): the index's effectiveMutationsThreshold at 5,000 vectors goes from 1,000 to 250 after that statement, and the engine refuses a SCOPE.SERVER setting there with a message that names SET SERVER SETTING.

Fix: a short "Database settings" section in how-to/databases-and-queries.adoc (the statement, the scope rule, how to read a vector setting back through get_stats()), the statement in the docstring, and a test that pins it on both wheels. No new Python API: DSL first.