Make comment about small bias in %.

Fixes rust-lang#16354.

src/doc/guide.md

@@ -1894,7 +1894,8 @@ Also, you may wonder why we are using `%` on the result of `rand::random()`.
 This operator is called 'modulo', and it returns the remainder of a division.
 By taking the modulo of the result of `rand::random()`, we're limiting the
 values to be between 0 and 99. Then, we add one to the result, making it from 1
-to 100.
+to 100. Using modulo can give you a very, very small bias in the result, but
+for this example, it is not important.
 
 Let's try to compile this using `cargo build`: