Rollup merge of #46884 - Manishearth:vec-docs, r=steveklabnik
Rollup merge of #46884 - Manishearth:vec-docs, r=steveklabnik

Clarify vec docs on deallocation (fixes #46879)

r? @steveklabnik

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