[api] Add documentation for the PropertyAttribute enum.

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BUG=v8:5260

Review-Url: https://codereview.chromium.org/2290553003
Cr-Commit-Position: refs/heads/master@{#38982}

include/v8.h

@@ -2673,11 +2673,17 @@ class V8_EXPORT Uint32 : public Integer {
   static void CheckCast(v8::Value* obj);
 };
 
-
+/**
+ * PropertyAttribute.
+ */
 enum PropertyAttribute {
-  None       = 0,
-  ReadOnly   = 1 << 0,
-  DontEnum   = 1 << 1,
+  /** None. **/
+  None = 0,
+  /** ReadOnly, i.e., not writable. **/
+  ReadOnly = 1 << 0,
+  /** DontEnum, i.e., not enumerable. **/
+  DontEnum = 1 << 1,
+  /** DontDelete, i.e., not configurable. **/
   DontDelete = 1 << 2
 };