DOC: improve readability of signature description (type hints)
## Problem description

I'm generally grateful for the efficient documentation of pandas, and I like the advancements in the new layout of recent versions, except for one thing:

The function/object signature at the very top is much harder to read now that the type annotations are printed as well. How about shading the type annotation in gray, or leaving it away altogether? It's enough if the type is described in the "Parameters" section IMHO. Also, the argument could be formatted differently than the default values. 

![image](https://user-images.githubusercontent.com/11406002/77593502-963ee000-6ef4-11ea-9a4b-164111f8f605.png)

I don't want to unleash a discussion about aesthetics, simply knock the issue away in case it doesn't comply with the guidelines. Thanks anyway!
