sql: Document system database

[sploiselle]

Briefly mentioned in #1160, but the tables of the system database should each be documented.

We don't see many questions about this currently (so I don't think it meets the 1.0 milestone requirement), but think it warrants documentation.

Currently the only reference to the system database is in Learn CockroachDB SQL, which says:

CockroachDB comes with a single default system database, which contains CockroachDB metadata and is read-only.

[rmloveland]

I think this should be internal engineering/support documentation. IIRC, users should not be doing anything with the system database other than knowing it exists and is used by the system. What do you think @sploiselle ? Can I convince you of this position? 🙂

[rmloveland]

users should not be doing anything with the system database

After reading some internal discussion about tables that live in system.* I realize this is not a correct opinion. :-)

[rmloveland]

E.g., DELETE FROM system.table_statistics WHERE name LIKE '%foo%';

[ericharmeling]

Closing in favor of #10340