Rollup merge of #38247 - federicomenaquintero:assert-eq-argument-docs, r=GuillaumeGomez
Rollup merge of #38247 - federicomenaquintero:assert-eq-argument-docs, r=GuillaumeGomez

Document the optional extra arguments to assert_eq!() / assert_ne!()

And clarify that those arguments in assert!() are in fact formattable.

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