Updated QSql{Query,Result}::bindValue doc to reflect the current state.

Since 5.0 it is possible to use one call to bindValue to bind
values to multiple placeholders with the same name.

Task-number: QTBUG-23360
Change-Id: Ic838150d25dd07bca7bc9e5d91ab3362a73833d6
Reviewed-by: Matt Newell <newellm@blur.com>
Reviewed-by: Mark Brand <mabrand@mabrand.nl>