Improve docs for "increase | decrease"

Indicate that 'property' and 'message' are optional.
Indicate that 'increase' and 'decrease' can receive a function which
returns the property to be checked.

lib/chai/core/assertions.js

@@ -1959,19 +1959,26 @@ module.exports = function (chai, _) {
   Assertion.addChainableMethod('changes', assertChanges);
 
   /**
-   * ### .increase(function)
+   * ### .increase(target[, property[, message]])
    *
    * Asserts that a function increases a numeric object property
    *
    *     var obj = { val: 10 };
    *     var fn = function() { obj.val = 15 };
    *     expect(fn).to.increase(obj, 'val');
    *
+   * It can also receive a function which returns the property
+   *
+   *     var obj = { val: 10 };
+   *     var fn = function() { obj.val = 5 };
+   *     var getVal = function() { return obj.val };
+   *     expect(fn).to.increase(getVal);
+   *
    * @name increase
    * @alias increases
    * @alias Increase
-   * @param {String} target
-   * @param {String} property name
+   * @param {String|Function} target
+   * @param {String} property name _optional_
    * @param {String} message _optional_
    * @namespace BDD
    * @api public
@@ -2016,19 +2023,26 @@ module.exports = function (chai, _) {
   Assertion.addChainableMethod('increases', assertIncreases);
 
   /**
-   * ### .decrease(function)
+   * ### .decrease(target[, property[, message]])
    *
    * Asserts that a function decreases a numeric object property
    *
    *     var obj = { val: 10 };
    *     var fn = function() { obj.val = 5 };
    *     expect(fn).to.decrease(obj, 'val');
    *
+   * It can also receive a function which returns the property
+   *
+   *     var obj = { val: 10 };
+   *     var fn = function() { obj.val = 5 };
+   *     var getVal = function() { return obj.val };
+   *     expect(fn).to.decrease(getVal);
+   *
    * @name decrease
    * @alias decreases
    * @alias Decrease
-   * @param {String} target
-   * @param {String} property name
+   * @param {String|Function} target
+   * @param {String} property name _optional_
    * @param {String} message _optional_
    * @namespace BDD
    * @api public