Update documentation on the rationale for ignoring cookiesEnabled

In [this issue](https://github.com/Modernizr/Modernizr/issues/666) it was reported that navigator.cookiesEnabled always returns ```true``` in IE9 but I did not find that to be the case

![screen shot 2014-05-22 at 12 04 37 pm](https://cloud.githubusercontent.com/assets/589564/3056643/f6c939f4-e1cc-11e3-8b5b-96386aefb9de.png)

To get it to return ```false``` I updated the cookie settings like so:
![screen shot 2014-05-22 at 12 05 02 pm](https://cloud.githubusercontent.com/assets/589564/3056654/1667bc0e-e1cd-11e3-94ba-285d9cd0e5aa.png)

However, I still think cookiesEnabled is unreliable since it does not do a good job of detecting nuanced cookie settings (first party vs third party, [site-specific exceptions](https://bugzilla.mozilla.org/show_bug.cgi?id=578534#c16)) reliably. For example, this setting in IE9, which for all intents and purposes is blocking cookies, returns ```true``` for cookiesEnabled

![screen shot 2014-05-22 at 12 06 37 pm](https://cloud.githubusercontent.com/assets/589564/3056710/c0173270-e1cd-11e3-8bb5-33d00ee0b147.png)



Just wanted to clarify the rationale in the comment so it's more accurate as to why we ignore this property. If anyone wants to re-word this, feel free.

Full Disclosure: These tests were done in one of the Windows 7 IE9 VMs that Microsoft distributes