Explain macro indirection

`offset_of!` implementation was moved to an internal macro
`_memoffset__offset_of_impl!` in
[PR #72](#72).

This was done in order to have a single top-level macro that
doc-comments can be attached to.

src/offset_of.rs

@@ -121,6 +121,8 @@ macro_rules! _memoffset__offset_of_impl {
 #[macro_export(local_inner_macros)]
 macro_rules! offset_of {
     ($parent:path, $field:tt) => {
+        // Macro implementation is delegated to another macro to have a
+        // single top-level macro to attach documentation to.
         _memoffset__offset_of_impl!($parent, $field)
     };
 }
@@ -196,6 +198,8 @@ macro_rules! _memoffset__offset_of_union_impl {
 #[macro_export(local_inner_macros)]
 macro_rules! offset_of_union {
     ($parent:path, $field:tt) => {{
+        // Macro implementation is delegated to another macro to have a
+        // single top-level macro to attach documentation to.
         _memoffset__offset_of_union_impl!($parent, $field)
     }};
 }