Rollup merge of #32183 - bluss:doc-index, r=alexcrichton
Rollup merge of #32183 - bluss:doc-index, r=alexcrichton

Clarify doc for slice slicing (Index impls)

Clarify doc for slice slicing (Index impls)

This is a follow up for PR #32099 and #32057

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