Align documentation with phpDoc for filter configs

1. The following config options were not properly documented in
   config_defaults_inc.php; the corresponding description was copied
   from the Admin Guide:

   • $g_filter_by_custom_fields
   • $g_filter_custom_fields_per_row
   • $g_view_filters

2. The default and minimum value for $g_filter_custom_fields_per_row
   was incorrectly documented as being 7 in the Admin guide; changed to
   8 to match value defined in config_defaults_inc.php (and matching
   actual usage in the code).