Add createStubInstance header in stubs.md

I kept scrolling past the parts of this page that explain `createStubInstance` so I created a section for it. I think the reason it didn't already exist is because it's considered a part of the utilities API, not the stub API. Even if that's the reasoning, I hope this PR is still a net positive, because personally, it makes it easier to find what I'm looking for.

docs/release-source/release/stubs.md

@@ -90,6 +90,8 @@ Note that it's usually better practice to stub individual methods, particularly
 
 Stubbing individual methods tests intent more precisely and is less susceptible to unexpected behavior as the object's code evolves.
 
+#### `var stubInstance = sinon.createStubInstance(MyConstructor, overrides);`
+
 If you want to create a stub object of `MyConstructor`, but don't want the constructor to be invoked, use this utility function.
 
 ```javascript