refactor(Assertion): small edit and improve docs

- Rename third parameter of Assertion constructor from `stack` to
  `ssfi` for consistency's sake.
- Add documentation to Assertion constructor explaining what the `object`,
  `message`, and `ssfi` flags are for.

lib/chai/assertion.js

@@ -26,11 +26,34 @@ module.exports = function (_chai, util) {
    *
    * Creates object for chaining.
    *
+   * `Assertion` objects contain metadata in the form of flags. Three flags can
+   * be assigned during instantiation by passing arguments to this constructor:
+   *
+   * - `object`: This flag contains the target of the assertion. For example, in
+   *   the assertion `expect(numKittens).to.equal(7);`, the `object` flag will
+   *   contain `numKittens` so that the `equal` assertion can reference it when
+   *   needed.
+   *
+   * - `message`: This flag contains an optional custom error message to be
+   *   prepended to the error message that's generated by the assertion when it
+   *   fails.
+   *
+   * - `ssfi`: This flag stands for "start stack function indicator". It
+   *   contains a function reference that serves as the starting point for
+   *   removing frames from the stack trace of the error that's created by the
+   *   assertion when it fails. The goal is to provide a cleaner stack trace to
+   *   end users by removing Chai's internal functions. Note that it only works
+   *   in environments that support `Error.captureStackTrace`, and only when
+   *   `Chai.config.includeStack` hasn't been set to `false`.
+   *
+   * @param {Mixed} obj target of the assertion
+   * @param {String} msg (optional) custom error message
+   * @param {Function} stack (optional) starting point for removing stack frames
    * @api private
    */
 
-  function Assertion (obj, msg, stack) {
-    flag(this, 'ssfi', stack || Assertion);
+  function Assertion (obj, msg, ssfi) {
+    flag(this, 'ssfi', ssfi || Assertion);
     flag(this, 'object', obj);
     flag(this, 'message', msg);