[libclang] Document IB attribute cursor kinds

[modocache]

Summary

Add documentation for Interface Builder attribute cursor kinds to
libclang public headers.

[compnerd]

Why add docs for these but not the others around it?

[modocache]

Because I happen to know what these kinds are! ;)

But I agree--they should all be documented. I can look into what the other kinds mean and add appropriate documentation for those as well. Maybe in another commit?

include/clang-c/Index.h

enum CXCursorKind {
   /* Attributes */
   CXCursor_FirstAttr                     = 400,
   /**
    * \brief An attribute whose specific kind is not exposed via this
    * interface.
    */
   CXCursor_UnexposedAttr                 = 400,
 
+  /**
+   * \brief Interface Builder's @IBAction attribute.
+   */
   CXCursor_IBActionAttr                  = 401,
+
+  /**
+   * \brief Interface Builder's @IBOutlet attribute.
+   */
   CXCursor_IBOutletAttr                  = 402,
+
+  /**
+   * \brief Interface Builder's @IBOutletCollection attribute.
+   */
   CXCursor_IBOutletCollectionAttr        = 403,
   CXCursor_CXXFinalAttr                  = 404,
   CXCursor_CXXOverrideAttr               = 405,
   CXCursor_AnnotateAttr                  = 406,
   CXCursor_AsmLabelAttr                  = 407,
   CXCursor_PackedAttr                    = 408,
   CXCursor_PureAttr                      = 409,
   CXCursor_ConstAttr                     = 410,