LibJS: Add spec comments and check for edge cases in Math.atanh

Author: https://github.com/davidot
Commit: c565cbd30c
Pull-request: https://github.com/SerenityOS/serenity/pull/16234

Userland/Libraries/LibJS/Tests/builtins/Math/Math.atanh.js

@@ -7,4 +7,6 @@ test("basic functionality", () => {
     expect(Math.atanh(0)).toBe(0);
     expect(Math.atanh(0.5)).toBeCloseTo(0.549306);
     expect(Math.atanh(1)).toBe(Infinity);
+    expect(Math.atanh(NaN)).toBe(NaN);
+    expect(Math.atanh(-0.0)).toBe(-0.0);
 });