Elaborate on naming conventions for `internal tables`

[kwannoel]

Where's the bug?

https://docs.risingwave.com/docs/current/sql-show-internal-tables

Describe the bug

https://docs.risingwave.com/docs/current/sql-show-internal-tables/

[kwannoel]

We need to explain their naming conventions, so users can be aware of how to use the table name as metadata, and to introspect the table info.