Rollup merge of #72986 - pickfire:vec-assert, r=Mark-Simulacrum
Rollup merge of #72986 - pickfire:vec-assert, r=Mark-Simulacrum

Add more assert to Vec with_capacity docs

Show assertion on len too to show them how adding new items will affect both the
length and capacity, before and after.

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