Rollup merge of #51014 - GuillaumeGomez:env_docs, r=QuietMisdreavus
Rollup merge of #51014 - GuillaumeGomez:env_docs, r=QuietMisdreavus

Add documentation about env! second argument

Fixes #48044.

r? @QuietMisdreavus

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