Improve inline documentation for the wpdb::$show_errors property.

Note that SQL/DB errors are displayed by default if both WP_DEBUG and WP_DEBUG_DISPLAY evaluate to true.

Props ericlewis.
Fixes #16615.


git-svn-id: https://develop.svn.wordpress.org/trunk@28243 602fd350-edb4-49c9-b593-d223f7449a82