js_exn: add brief explanations and links to MDN.

[jdeisenberg]

Please check the documentation for asJsExn; I am not sure I read the code for this correctly (nor am i sure how to use it).

[ryyppy]

Suggested change

	Returns `Some(exn)` as a JavaScript exception if its argument is an OCaml `Error`, `None` otherwise. *Someone needs to check this; I am not really sure this is an accurate description.*
	Returns `Some(jsExn)` if the input argument `exn` is an actual JS exception, and returns `None` if `exn` is a native Reason exception.
	```re example
	let throwJsException: unit => 'a = [%raw {|function() { throw "hello"; }|}];
	// This will log out "hello", since a JS exception was thrown
	try (throwJsException()) {
	| myExn => switch(myExn->Js.Exn.asJsExn) {
	| Some(jsExn) => jsExn->Js.log
	| None => Js.log("this is a Reason exception")
	}
	};
	exception MyError;
	// This will log out "this is a Reason exception", since our raised error is a native Reason exception
	try (raise(MyError)) {
	| myExn => switch(myExn->Js.Exn.asJsExn) {
	| Some(jsExn) => jsExn->Js.log
	| None => Js.log("this is a Reason exception")
	}
	};
	```

(Thanks to @cristianoc for explaining this feature)

[ryyppy]

after further discussion, this function seems rather redundant, since the same functionality can be achieved via pattern matching:

try(throwJsException()) {
  | Js.Exn.Error(_) => Js.log("Js exception");
  | _ => Js.log("Some Reason exception")
}

[jdeisenberg]

How about combining these? Change the explanation to:

Returns Some(jsExn) if the input argument exn is an actual JS exception, and returns None if exn is a native Reason exception.

and then add:

Rather than using this function, you can get the same functionality with pattern matching:

with your example.

[ryyppy]

yeah, that sounds good!