Improve tooltip docs

https://datalore.jetbrains.com/report/static/HZqq77cegYd.E7get_WnChZ/y8xk12VJ0sHpRHcWnrDhQO

> field (string): The name of the variable/aesthetics. The field name begins with ^ for aesthetics. You can specify variable names without a prefix, but the @ prefix can be also used. It's possible to set a format for all positional aesthetics: ^X (all positional x) and ^Y (all positional y). For example:
>
>field = '^Y' - for all positional y;
>field = '^y' - for y aesthetic;
>field = 'y' - for variable with the name "y".

"field = '^Y' - for all positional y;"
let's change to:
"field = '^Y' - for all Y-axis aesthetic, e.g. `y`, `ymin`, `ymax` etc.;"

Also I can see a strange line on the left of sone sections in the doc: