QtSql: Add alt texts

qdoc: 'QtSql': \image books-demo.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtSql': \image cachedtable-example.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtSql': \image drilldown-example.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtSql': \image masterdetail-example.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtSql': \image querymodel-example.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtSql': \image relationaltablemodel-example.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtSql': \image sqlbrowser-demo.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtSql': \image sql-widget-mapper.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtSql': \image widgetmapper-sql-mapping-table.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtSql': \image widgetmapper-sql-mapping.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtSql': \image tablemodel-example.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtSql': \image qdatawidgetmapper-simple.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtSql': \image relationaltable.png is without a textual description, QDoc will not generate an alt text for the image.