Make `IndexMut` note apply to more cases
Make `IndexMut` note apply to more cases

Previously it was only emitted for assigments, which was an
unnecessary restriction. Now it doesn't care where the mutability
comes from.

This commit also adds `` quotes around the printed type.

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