Add documentation for transparent_union attribute

[arphaman]

Summary

This patch adds a bit of documentation for the transparent_union attribute.

[aaron.ballman]

LGTM, thank you for the improved docs!

cfe/trunk/include/clang/Basic/Attr.td

continue;
       return Ret;
     }
   }];
 }
 
 def TransparentUnion : InheritableAttr {
   let Spellings = [GCC<"transparent_union">];
 //  let Subjects = SubjectList<[Record, TypedefName]>;
-  let Documentation = [Undocumented];
+  let Documentation = [TransparentUnionDocs];
   let LangOpts = [COnly];
 }
 
 def Unavailable : InheritableAttr {
   let Spellings = [GNU<"unavailable">];
   let Args = [StringArgument<"Message", 1>,
               EnumArgument<"ImplicitReason", "ImplicitReason",
                 ["", "", "", ""],

cfe/trunk/include/clang/Basic/AttrDocs.td

def XRayDocs : Documentation {
   let Content = [{
 ``__attribute__((xray_always_instrument))`` or ``[[clang::xray_always_instrument]]`` is used to mark member functions (in C++), methods (in Objective C), and free functions (in C, C++, and Objective C) to be instrumented with XRay. This will cause the function to always have space at the beginning and exit points to allow for runtime patching.
 
 Conversely, ``__attribute__((xray_never_instrument))`` or ``[[clang::xray_never_instrument]]`` will inhibit the insertion of these instrumentation points.
 
 If a function has neither of these attributes, they become subject to the XRay heuristics used to determine whether a function should be instrumented or otherwise.
   }];
 }
+
+def TransparentUnionDocs : Documentation {
+  let Category = DocCatType;
+  let Content = [{
+This attribute can be applied to a union to change the behaviour of calls to
+functions that have an argument with a transparent union type. The compiler
+behaviour is changed in the following manner:
+
+- A value whose type is any member of the transparent union can be passed as an
+argument without the need to cast that value.
+
+- The argument is passed to the function using the calling convention of the
+first member of the transparent union. Consequently, all the members of the
+transparent union should have the same calling convention as its first member.
+
+Transparent unions are not supported in C++.
+  }];
+}