MakeCurrent no longer generates EGL_BAD_SURFACE when the pbuffers are
IOSurfaces. So the documentation has been updated to match.

Bug chromium:987289

Change-Id: Id074d9d6d8d0a50c91ec725ec2016a4f5ad4b6e4
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/34568
Presubmit-Ready: Sean Risser <srisser@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Sean Risser <srisser@google.com>