Config+Doc: Move 'Database Tables' section to 'Database'

Since the introduction of db_get_table() and removal of database table
name config options, the 'Database Table Variables' section is mostly
empty, containing only the table prefix and suffix. Since these
configuration are related to database settings, it makes sense to merge
the 2 sections.

$g_db_table_prefix, $g_db_table_suffix and $g_db_table_plugin_prefix
configs are now defined under the 'Database Settings' section in
config_defaults_inc.php.

Fixes 0022221