document: Update [Unforgeable] document: not applicable on interfaces

Our IDLExtendedAttributes said that [Unforgeable] is applicable on
interfaces, but it is not true, and we don't have a tag "(i)".
This CL removes the description about the application on interfaces.

Bug: None
Change-Id: I7c43b11f5f074ba89c9e497845a72696f403db43
Reviewed-on: https://chromium-review.googlesource.com/1195248
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Hitoshi Yoshida <peria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587035}

third_party/blink/renderer/bindings/IDLExtendedAttributes.md

@@ -506,17 +506,16 @@ Implementation: Given `[TreatNullAs=EmptyString]`, a JavaScript null is converte
 
 Standard: [Unforgeable](http://heycam.github.io/webidl/#Unforgeable)
 
-Summary: Makes interface members unconfigurable and also controls where the member is defined.
+Summary: Makes attributes or non-static oprations unconfigurable and also controls where the member is defined.
 
-Usage: Can be specified on methods, attributes or interfaces:
+Usage: Can be specified on methods or non-static attributes:
 
 ```webidl
 [Unforgeable] void func();
 [Unforgeable] attribute DOMString str;
-[Unforgeable] interface XXX { ... };
 ```
 
-By default, interface members are configurable (i.e. you can modify a property descriptor corresponding to the member and also you can delete the property). `[Unforgeable]` makes the member unconfiguable so that you cannot modify or delete the property corresponding to the member.
+By default, members are configurable (i.e. you can modify a property descriptor corresponding to the member and also you can delete the property). `[Unforgeable]` makes the member unconfiguable so that you cannot modify or delete the property corresponding to the member.
 
 `[Unforgeable]` changes where the member is defined, too. By default, attribute getters/setters and methods are defined on a prototype chain. `[Unforgeable]` defines the member on the instance object instead of the prototype object.