Note naming convention of lists (xs, ys, ...)

People reading the tutorial may not be familiar with the convention of naming lists, vectors and the like as xs, ys, etc. Without some explanation of the reasoning behind it, it might come off as just throwaway non-descriptive names. Languages like Haskell gets flak from using short, non-descriptive names, while in reality, there are clear conventions and reasons for using certain terse variable names.

I assumed that the convention came from a language like Haskell, so I
tailored the explanation according to that.
auto merge of #15410 : LemmingAvalanche/rust/patch-1, r=alexcrichton

People reading the tutorial may not be familiar with the convention of naming lists, vectors and the like as xs, ys, etc. Without some explanation of the reasoning behind it, it might come off as just throwaway non-descriptive names. Languages like Haskell gets flak from using short, non-descriptive names, while in reality, there are clear conventions and reasons for using certain terse variable names. 

This is just a proposed explanation of this convention, as I've interpreted it - I assumed that the convention came from a language like Haskell, so I tailored it according to that. So beware that I might have misjudged how it is used in the Rust language, or at least how it is used in the Rust tutorial.