Rollup merge of #133790 - HypheX:improve-vec-docs, r=harudagondi,workingjubilee

Improve documentation for Vec::extend_from_within

This closes #104762.

It rephrases some of the explanations, and greatly improves the clarity of the example.
Based on this PR and its discussions:
https://github.com/rust-lang/rust/pull/105030/files#r1059808792

Author: matthiaskrgr