docs: update SELECT command for redis-databases support

[hanxi]

No description provided.

[git-hulk]

Suggested change

	The `SELECT` command is a placeholder by default (`redis-databases = 0`). When `redis-databases` > 0, it switches between databases (0 to redis-databases-1). Database 0 uses the default namespace, databases 1-N use `db1`, `db2`, etc. Enabling this disables custom namespaces. Use [the namespace feature](https://kvrocks.apache.org/docs/namespace) for multiple isolated databases without `redis-databases`.
	In Kvrocks the `SELECT` command is just a placeholder and does not switch between dbs if the `redis-database` is not enabled(`redis-databases = 0` by default).
	A [the namespace feature](https://kvrocks.apache.org/docs/namespace) instead if multiple dbs are needed.
	Before [NEXT RELEASE VERSION], Kvrocks only allowed using [the namespace feature](https://kvrocks.apache.org/docs/namespace) to isolate your data. That said, the `SELECT` command is just a placeholder and does not switch between databases. And now, the Redis database mechanism is supported, and you can enable it by setting the `redis-database` to the database number you expect in the configuration file.
	To be noticed, we don't allow using the namespace feature and the Redis database at the same time.

[git-hulk]

Suggested change

	| SELECT | ✓ | unstable | Switches between databases when `redis-databases` > 0 (default 0: returns OK without switching). |
	| SELECT | ✓ | v1.0.0 | Switches between databases when `redis-databases` > 0 (default 0: returns OK without switching) since [NEXT RELEASE VERSION]. |

[git-hulk]

@hanxi We can put this change on hold until the next future version is released. cc @PragmaTwice