Specify the result of integer cast on boolean

Author: GuillaumeGomez

src/libstd/primitive_docs.rs

@@ -12,7 +12,8 @@
 //
 /// The boolean type.
 ///
-/// The `bool` represents a value, which could only be either `true` or `false`.
+/// The `bool` represents a value, which could only be either `true` or `false`. If you cast
+/// a `bool` into an integer, `true` will be 1 and `false` will be.
 ///
 /// # Basic usage
 ///