Specify bit validity and padding of some types

src/type-layout.md

@@ -56,6 +56,18 @@ Most primitives are generally aligned to their size, although this is
 platform-specific behavior. In particular, on x86 u64 and f64 are only
 aligned to 32 bits.
 
+For every primitive numeric type (`u8`, `i8`, `u16`, `i16`, `u32`, `i32`, `u64`,
+`i64`, `u128`, `i128`, `usize`, `isize`, `f32`, and `f64`), `T`, the bit validity
+of `T` is equivalent to the bit validity of `[u8; size_of::<T>()]`. `u8` has 256
+valid representations (namely, every 8-bit sequence). An uninitialized byte is not
+a valid `u8`. A byte at any offset in a reference or pointer type may not be a
+valid `u8` (the semantics of transmuting a reference or pointer to a non-pointer
+type is currently undecided).
+
+For the primitive numeric types and also for `bool` and `char`, every byte is
+guaranteed to be initialized (in other words, for every such type, `T`,
+`transmute::<T, [u8; size_of::<T>()]>(...)` is always sound).
+
 ## Pointers and References Layout
 
 Pointers and references have the same layout. Mutability of the pointer or