Auto merge of #6340 - giraffate:improve_doc_for_map_clone, r=Manishearth
Auto merge of #6340 - giraffate:improve_doc_for_map_clone, r=Manishearth

Improve doc about `map_clone`

A follow up of https://github.com/rust-lang/rust-clippy/issues/6239#issuecomment-719100677.

`map_clone` works with not only `Iterator` but `Option` although not written in [doc](https://rust-lang.github.io/rust-clippy/master/#map_clone). Also, an example in the doc shows  a usage of dereferencing, but this isn't also written.

changelog: Improve doc about `map_clone`

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