Add details about types with interior mutability to E0386.
Add details about types with interior mutability to E0386.

Types with interior mutability like `Cell` and `RefCell` can be used to
skirt the restriction on mutating mutable values inside an immutable
container.

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