[PATCH] Add AttrDocs for objc_boxable attribute

[John McCall]

> On Jul 23, 2015, at 1:20 PM, AlexDenisov <1101.debian at gmail.com> wrote:
> Patch adds rough documentation for objc_boxable attribute

+def ObjCBoxableDocs : Documentation {
+    let Category = DocCatType;
+    let Content = [{
+Structs and unions marked with ``objc_boxable`` attribute can be used inside of boxed expressions.

“Structs and unions marked with the ``objc_boxable`` attribute can be used with the Objective-C boxed expression syntax, ``@(…)``.”

+**Usage**: ``__attribute__((objc_boxable))``.  This attribute
+can only be placed at record declaration or record definition:

This should be a separate paragraph; just add an empty line before **Usage**.

“This attribute can only be placed on a declaration of a trivially-copyable struct or union:”

+
+.. code-block:: objc
+
+  struct __attribute__((objc_boxable)) some_struct {
+    int i;
+  };
+  union __attribute__((objc_boxable)) some_union {
+    int i;
+    float f;
+  };
+  typedef struct __attribute__((objc_boxable)) _some_struct some_struct;
+
+  // ...
+
+  some_struct ss;
+  NSValue *boxed = @(ss);

John.