mw.notification: Re-implement autoHideSeconds as string preset option

mw.notification: Re-implement autoHideSeconds as string preset option

Instead of allowing freeform number values, make it a preset-based
string where pre-defined values are encouraged.

A numerical override is still possible by creating a new preset,
but this undocumented/private for now.

Follows-up 4b9992a0, which added this option. No usage found
as of yet in Wikimedia Git, thus no deprecation/back-compat.

Also:
* Unbreaks `#autoHideSeconds` reference in #autoHideLimit doc.
* Improve wording of #autoHideLimit doc.

Bug: T155228
Change-Id: Id84d8b616fe26b9976263f4bd6bdc70cc8a539ba