Clarified that let(mut x, y) = only makes x mutable, not y

src/doc/book/mutability.md

@@ -55,6 +55,8 @@ fn foo(mut x: i32) {
 # }
 ```
 
+Note that here, the `x` is mutable, but not the `y`.
+
 [pattern]: patterns.html
 
 # Interior vs. Exterior Mutability