Update docs WRT repr(Rust) (#69)

Issue:
 - https://github.com/Gilnaa/memoffset/issues/59

src/offset_of.rs

@@ -86,6 +86,16 @@ macro_rules! _memoffset_offset_from_unsafe {
 ///     assert_eq!(offset_of!(Foo, b), 4);
 /// }
 /// ```
+///
+/// ## Notes
+/// Rust's ABI is unstable, and [type layout can be changed with each
+/// compilation](https://doc.rust-lang.org/reference/type-layout.html).
+///
+/// Using `offset_of!` with a `repr(Rust)` struct will return the correct offset of the
+/// specified `field` for a particular compilation, but the exact value may change
+/// based on the compiler version, concrete struct type, time of day, or rustc's mood.
+///
+/// As a result, the value should not be retained and used between different compilations.
 #[macro_export(local_inner_macros)]
 macro_rules! offset_of {
     ($parent:path, $field:tt) => {{

src/span_of.rs

@@ -52,11 +52,18 @@ macro_rules! _memoffset__compile_error {
 /// span_of!(Struct, start ..)
 /// ```
 ///
-/// *Note*:
+/// ### Note
 /// This macro uses recursion in order to resolve the range expressions, so there is a limit to
 /// the complexity of the expression.
 /// In order to raise the limit, the compiler's recursion limit should be lifted.
 ///
+/// ### Safety
+/// The inter-field form mentioned above assumes that the first field is positioned before the
+/// second.
+/// This is only guarenteed for `repr(C)` structs.
+/// Usage with `repr(Rust)` structs may yield unexpected results, like downward-going ranges,
+/// spans that include unexpected fields, empty spans, or spans that include *unexpected* padding bytes.
+///
 /// ## Examples
 /// ```
 /// use memoffset::span_of;