Document database limit (0–15) for SELECT command as a note (valkey-io#339)

Addresses issue valkey-io#338. 

The note is inserted after the sentence "New connections always use the
database 0." to ensure it's visible early in the section.

As this is my first PR please let me know if you'd like this phrased
differently, thank you.

---------

Signed-off-by: Dragos Andriciuc <andriciucdragos@protonmail.com>
Signed-off-by: Viktor Söderqvist <viktor.soderqvist@est.tech>
Co-authored-by: Viktor Söderqvist <viktor.soderqvist@est.tech>

Author: Andriciuc

commands/select.md

@@ -1,6 +1,10 @@
 Select the Valkey logical database having the specified zero-based numeric index.
 New connections always use the database 0.
 
+By default, Valkey provides 16 logical databases, indexed from `0` to `15`.
+You can increase this number by setting the `databases` parameter in the configuration file.
+This change must be made at startup and cannot be modified at runtime.
+
 Selectable Valkey databases are a form of namespacing: all databases are still persisted in the same RDB / AOF file. However different databases can have keys with the same name, and commands like [`FLUSHDB`](flushdb.md), [`SWAPDB`](swapdb.md) or [`RANDOMKEY`](randomkey.md) work on specific databases.
 
 In practical terms, Valkey databases should be used to separate different keys belonging to the same application (if needed), and not to use a single Valkey instance for multiple unrelated applications.