docs; Date method caveats

closes Automattic#1598

Author: aheckmann

docs/schematypes.jade

@@ -79,6 +79,20 @@ block content
     m.save(callback);
 
   h3 Usage notes:
+  h4#Dates Dates
+  :markdown
+    [Built-in `Date` methods](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date) are [__not__ hooked into](https://github.com/LearnBoost/mongoose/issues/1598) the mongoose change tracking logic which in English means that if you use a `Date` in your document and modify it with a method like `setMonth()`, mongoose will be unaware of this change and `doc.save()` will not persist this modification. If you must modify `Date` types using built-in methods, tell mongoose about the change with `doc.markModified('pathToYourDate')` before saving.
+
+  :js
+    var Assignment = mongoose.model('Assignment', { dueDate: Date });
+    Assignment.findOne(function (err, doc) {
+      doc.dueDate.setMonth(3);
+      doc.save(callback) // THIS DOES NOT SAVE YOUR CHANGE
+
+      doc.markModified('dueDate');
+      doc.save(callback) // works
+    })
+
   h4#mixed Mixed
   p An "anything goes" SchemaType, its flexibility comes at a trade-off of it being harder to maintain. Mixed is available either through Schema.Types.Mixed or by passing an empty object literal. The following are equivalent:
   :js