Doc: Improve SQL Example: SQL Widget Mapper Example

• The application is buggy (at least when running from Creator 4.6.1/Qt 5.10.1 on Linux). The values in the Type field act in a weird way, e.g. changing the first entry from Home to Other, then going back and forth in the list of entries using Next*and *Previous, sets this entry type to Work.
• The text description seems to be written as the continuation of a different Widget example, and not as a SQL example. The text contains implicit references to this other material.

See also:

• https://wiki.qt.io/Examples_Revamping_Qt_SQL#Notes
• https://wiki.qt.io/Examples_Revamping_Qt_SQL#Examples