QDoc: Introduce NodeText::GrammaticalNumber

This change adds a typed/scoped enum for grammatical numbers to the
NodeText namespace. Its intended use is to replace the bool parameter of
NodeText::typeString in a follow-up refactoring.

Task-number: QTBUG-151328
Pick-to: 6.12
Change-Id: I711b0111dda294c4eed661cf518cff0288004d9c
Reviewed-by: Topi Reinio <topi.reinio@qt.io>
Reviewed-by: David Boddie <david.boddie@qt.io>