Update boolean.md

Author: joshlf

src/types/boolean.md

@@ -92,6 +92,12 @@ boolean type for its operands, they evaluate using the rules of [boolean logic].
 * `a < b` is the same as `!(a >= b)`
 * `a <= b` is the same as `a == b | a < b`
 
+## Bit validity
+
+The single byte of a `bool` is guaranteed to be initialized (in other words,
+`transmute::<bool, u8>(...)` is always sound -- but since some bit patterns
+are invalid `bool`s, the inverse is not always sound).
+
 [boolean logic]: https://en.wikipedia.org/wiki/Boolean_algebra
 [enumerated type]: enum.md
 [expressions]: ../expressions.md