Rollup merge of #80442 - steffahn:mention_arc_in_cow, r=Mark-Simulacrum
Rollup merge of #80442 - steffahn:mention_arc_in_cow, r=Mark-Simulacrum

Mention Arc::make_mut and Rc::make_mut in the documentation of Cow

Following this discussion: https://users.rust-lang.org/t/should-the-cow-documentation-mention-arc/53341

_Rendered (the last paragraph is new):_

![Screenshot_20201228_171551](https://user-images.githubusercontent.com/3986214/103228135-5d72e200-4930-11eb-89e1-38b5c86b08c7.png)

`@rustbot` modify labels: T-doc, T-libs

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