reference documentation does not explain "if let" and "while let" clearly

[froydnj]

When looking at the reference docs (stable and nightly), the sections on "if let expressions" and "while let expressions" both refer to "refutable let statements". I think there are two problems with this description:

1. The phrase "let statements" only appears in the section on "type coercions", and even there its meaning is only hinted at by example. Maybe this should somehow refer to the section on "declaration statements" (which itself doesn't currently mention let)?
2. The phrase "refutable let statements" is nowhere defined in the document. The section on functions mentions "function arguments are irrefutable patterns", but the notion of refutability doesn't get a good treatment. There are examples, which do help, but this is not necessarily the same thing as explaining it in prose, and a prose description would provide a place to hyperlink to.

(All this being said, I did clear up my own confusion on this by being able to look at the examples, which are super-helpful.)