Improve documentation of some ItemList signals

[timothyqiu]

• When mouse position is mentioned, it's better to specify the coordinate system being used.
• Clarify that item_selected is only applicable in single selection mode. i.e., multi-selecting an item won't emit both item_selected and multi_selected.

[Mickeon]

Good changes, but these descriptions smell old. If we really want to change them for the better they could be reworded outright, for example by saying "Emitted", not "Triggered".

[timothyqiu]

Done. Also changed the remaining "Triggered" to "Emitted" since there are just two of them.

[akien-mga]

Thanks!