Rename Mentions sub-toggle to reflect better the actual targeted events in Notifications Preferences

Consider the following use-case:

On an instance there are Admin and 2 users: U1 and U2, all with Mentions toggles set to ON in Notifications Preferences.

Admin mentions U1 in Sandbox page, U1 gets 2 events (his mention and the Sandbox page edit event), but U2 gets only the Sandbox page edit event, although he also has Mentions set to ON.

Basically the Mentions toggle (ON/OFF) is only for mentions of own user at the moment, not for receiving notifications about third party users mentions.

Since Mentions sub-toggle is also named 'Mentions' it may create confusion in this respect and may help if it can be named 'Mentions for own user' for example.