Documented the #[repr(align(x))] attribute.

src/attributes.md

@@ -142,10 +142,14 @@ On `enum`s:
 On `struct`s:
 
 - `repr` - specifies the representation to use for this struct. Takes a list
-  of options. The currently accepted ones are `C` and `packed`, which may be
-  combined. `C` will use a C ABI compatible struct layout, and `packed` will
-  remove any padding between fields (note that this is very fragile and may
-  break platforms which require aligned access).
+  of options. The currently accepted ones are `C`, `align` and `packed`.
+  `C` will use a C ABI compatible struct layout. `C` can be used in conjunction
+  with `align` or `packed`.  `#[repr(align(x))]` takes one argument which
+  increases alignment of the `struct` to the given value. The alignment must be
+  an unsigned power of two. `packed` will remove any padding between
+  fields (note that this is very fragile and may break platforms which require
+  aligned access). `align` and `packed` cannot be applied on the same struct,
+  and a `packed` struct cannot transitively contain another struct with `align`.
 
 On `union`s:
 
@@ -416,4 +420,4 @@ You can implement `derive` for your own type through [procedural macros].
 
 [Doc comments]: comments.html#doc-comments
 [The Rustdoc Book]: ../rustdoc/the-doc-attribute.html
-[procedural macros]: procedural-macros.html
+[procedural macros]: procedural-macros.html