Documentation/explaination for HD3000 ReservedMemory patch?

Hi there,

I've been getting things ready to install High Sierra using OpenCore on my ThinkPad X220, and noticed the following section in the [Laptop Sandy Bridge page](https://dortania.github.io/OpenCore-Install-Guide/config-laptop.plist/sandy-bridge.html#reservedmemory) of the Dortania OpenCore guide:

> **ReservedMemory**
>
> Used for exempting certain memory regions from OSes to use, mainly relevant for Sandy Bridge iGPUs or systems with faulty memory. Use of this quirk is not covered in this guide

Looking at the OpenCore 0.7.3 sample.plist, I found the following section which was added in a commit by @vit9696:

```
ReservedMemory


Address
268435456
Comment
HD3000: IGPU memory corruption errata
Enabled

Size
268435456
Type
Reserved

```

I've read a lot about the occasional freezing and glitch issues with the HD3000 on 10.13, and until now I'd never heard anyone figure out a concrete fix. If a user enables the above patch, is that supposed to fix those issues? If so, are the address and size values supposed to be customized base on the amount of RAM the user has installed?

If the above patch is indeed supposed to solve those issues, it would be great to have a short explanation and a recommendation to enable the patch in the Sandy Bridge guides. If it fixes some unrelated issue, it might be good to specify so future users don't get confused.

Thanks in advance, and thanks for all your hard work helping macOS run smoothly on non-Apple hardware!