Attempt to clarify use of `where i32: ConvertTo<T>
Attempt to clarify use of `where i32: ConvertTo<T>

I read this section a few times before even having a guess what
was meant, then consulted IRC for confirmation.  It may be that I
was thick-headed, but I think this is a useful addition.

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