Clarify documentation for string slicing (Index impls)
Clarify documentation for string slicing (Index impls)

- Mention the sugared syntax for the implementations, since it's not
  apparent from the docs that `Index<Range<usize>>` corresponds to
  `&self[a..b]`.
- Be specific in that start <= end and end <= len

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