Documentation for sdl2::video (#1487)

* Documentation for `sdl2::video`

Provided documentation for most of the `sdl2::video` functions.

Mostly did that to resolve some uncertanties in functions purpose (`grab`/`keyboard_grab`/`mouse_grab`, `minimize`).

Looked at `SDL_SetWindowData`/`SDL_GetWindowData` ToDo: think that those functions should be removed.

* Remove trailing `///` after comments

* Checked 'ToDo's

Author: JktuJQ