Get rid of "[+] show undocumented items" toggle on numeric From impls
Get rid of "[+] show undocumented items" toggle on numeric From impls

On most From implementations, the docstring is attached to the
function. This is also how people have been [recommended] to do it.

Screenshots:

* [before](https://user-images.githubusercontent.com/1593513/115767662-323c5480-a35e-11eb-9918-98aba83e9183.png)
* [after](https://user-images.githubusercontent.com/1593513/115767675-35374500-a35e-11eb-964f-c28eeb6c807a.png)

[recommended]: https://github.com/rust-lang/rust/issues/51430#issuecomment-398322434

graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust