Doc: Renamed generated list keyword.

-"qmlclasses" was used in Qt 4.7
-enables the listing of QML types in a page

Task-number: QTBUG-31490
Change-Id: I4f666945067ef1df6e358c488c245792b5aea3ab
Reviewed-by: Martin Smith <martin.smith@digia.com>
don't pass -indexdir during prepare_docs phase

the forward-referenced directories don't exist yet, so we get pointless
warnings. in fact, this is why we do a multi-pass build in the first
place, and consequently using indexes during the first pass is
illogical.

Task-number: QTBUG-32152
Change-Id: I66bf6b43238827e87cb8bf6932d581b808c1032d
Reviewed-by: Martin Smith <martin.smith@digia.com>