Doc: Don't show a description for omitted SH_ComboBox_UseNativePopup enum item

\omitvalue does not allow a description of an enum item, so move
the description as a comment to the enum declaration.

Change-Id: I4192b16e41b704cbad66c0eeafcb141087d2ba65
Reviewed-by: Martin Smith <martin.smith@digia.com>