Update the documentation of Isolate::GetCodeRange

On Win64, the first page is reserved for the embedder.

BUG=v8:3597
R=svenpanne@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/619233003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24383 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Change minimum code range size to 3MB

There will be at least 2MB of guard pages around allocatable code memory

BUG=none
R=svenpanne@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/617103006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24384 ce2b1a6d-e550-0410-aec6-3dcde31c8c00