fix(lib/runnable.js): Fix timeout upper limit check (again)

Seems PR mochajs#1652 upper limit check was off by one. Fixed it and documentation.

lib/runnable.js

@@ -61,7 +61,7 @@ utils.inherits(Runnable, EventEmitter);
  *
  * @description
  * A string argument can use shorthand (such as "2s") and will be converted.
- * If the value is `0` or exceeds `2^<sup>31</sup>`, timeouts will be disabled.
+ * If value is `0` or exceeds `2^<sup>31</sup>-1`, timeouts will be disabled.
  *
  * @private
  * @param {number|string} ms - Timeout threshold value.
@@ -76,7 +76,7 @@ Runnable.prototype.timeout = function(ms) {
     ms = milliseconds(ms);
   }
   // see #1652 for reasoning
-  if (ms === 0 || ms > Math.pow(2, 31)) {
+  if (ms === 0 || ms > Math.pow(2, 31) - 1) {
     this._enableTimeouts = false;
   }
   debug('timeout %d', ms);