Rollup merge of #76303 - jyn514:vec-assert-doc, r=Dylan-DPC
Rollup merge of #76303 - jyn514:vec-assert-doc, r=Dylan-DPC

Link to `#capacity-and-reallocation` when using with_capacity

Follow up to https://github.com/rust-lang/rust/pull/76058#discussion_r479655750.
r? @pickfire

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