[DOC] Documentation for flags of RObject

Merged: https://github.com/ruby/ruby/pull/7684

object.c

@@ -42,6 +42,18 @@
 #include "builtin.h"
 #include "shape.h"
 
+/* Flags of RObject
+ *
+ * 1:    ROBJECT_EMBED
+ *           The object has its instance variables embedded (the array of
+ *           instance variables directly follow the object, rather than being
+ *           on a separately allocated buffer).
+ * if !SHAPE_IN_BASIC_FLAGS
+ * 4-19: SHAPE_FLAG_MASK
+ *           Shape ID for the object.
+ * endif
+ */
+
 /*!
  * \addtogroup object
  * \{