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

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

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