Rollup merge of #42011 - rap2hpoutre:patch-5, r=GuillaumeGomez

improve collapse toggle render (css)

The `[-]` toggle for functions in docs _seems_ too big. It's just an impression, but it's something I noticed long time ago (maybe I have bad taste). I never thought to fix it, but, today I think: "Ok, why not suggest it.". Feel free to close without explanation!

Preview changes below:

From this:

<img width="1003" alt="capture d ecran 2017-05-15 a 17 14 45" src="https://cloud.githubusercontent.com/assets/1575946/26064816/5c84de86-3992-11e7-976b-41c625cace0f.png">

To this:

<img width="996" alt="capture d ecran 2017-05-15 a 17 15 02" src="https://cloud.githubusercontent.com/assets/1575946/26064854/78325dac-3992-11e7-88f6-2c43db43421c.png">