ScrolledWindow: Annotate h|vadjustment as nullable

ScrolledWindow: Annotate h|vadjustment as nullable

The code interprets NULL to mean 'create a new Adjustment and use that,
deposing the old one', but we neither documented nor annotated that
ability, so users could be unsure how to "unlink" a specific Adjustment.
While users could pass their own Adjustment in if this wasn't nullable,
we already support NULL in the code, and it doesn't hurt to document
that this is available as a convenience while retaining the behaviour.