Add documentation for FunctionCallbackInfo

R=verwaest@chromium.org,haraken@chromium.org,yukishiino@chromium.org
BUG=

Change-Id: I273f5ce305f80b2aa5e9c8c42a6e8e5afc51a0a7
Reviewed-on: https://chromium-review.googlesource.com/484422Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44927}

include/v8.h

@@ -3599,16 +3599,34 @@ class ReturnValue {
 template<typename T>
 class FunctionCallbackInfo {
  public:
+  /** The number of available arguments. */
   V8_INLINE int Length() const;
+  /** Accessor for the available arguments. */
   V8_INLINE Local<Value> operator[](int i) const;
   V8_INLINE V8_DEPRECATED("Use Data() to explicitly pass Callee instead",
                           Local<Function> Callee() const);
+  /** Returns the receiver. This corresponds to the "this" value. */
   V8_INLINE Local<Object> This() const;
+  /**
+   * If the callback was created without a Signature, this is the same
+   * value as This(). If there is a signature, and the signature didn't match
+   * This() but one of its hidden prototypes, this will be the respective
+   * hidden prototype.
+   *
+   * Note that this is not the prototype of This() on which the accessor
+   * referencing this callback was found (which in V8 internally is often
+   * referred to as holder [sic]).
+   */
   V8_INLINE Local<Object> Holder() const;
+  /** For construct calls, this returns the "new.target" value. */
   V8_INLINE Local<Value> NewTarget() const;
+  /** Indicates whether this is a regular call or a construct call. */
   V8_INLINE bool IsConstructCall() const;
+  /** The data argument specified when creating the callback. */
   V8_INLINE Local<Value> Data() const;
+  /** The current Isolate. */
   V8_INLINE Isolate* GetIsolate() const;
+  /** The ReturnValue for the call. */
   V8_INLINE ReturnValue<T> GetReturnValue() const;
   // This shouldn't be public, but the arm compiler needs it.
   static const int kArgsLength = 8;
@@ -5757,9 +5775,13 @@ class V8_EXPORT ObjectTemplate : public Template {
   friend class FunctionTemplate;
 };
 
-
 /**
  * A Signature specifies which receiver is valid for a function.
+ *
+ * A receiver matches a given signature if the receiver (or any of its
+ * hidden prototypes) was created from the signature's FunctionTemplate, or
+ * from a FunctionTemplate that inherits directly or indirectly from the
+ * signature's FunctionTemplate.
  */
 class V8_EXPORT Signature : public Data {
  public: