Clarify you cannot call Core.Graph.FindReferencedModule in a patch

[markfinal]

For example, this is bad, and results in a Null deference exception

        this.PublicPatch((settings, appliedTo) =>
            {
                if (this.BuildEnvironment.Platform.Includes(Bam.Core.EPlatform.Windows))
                {
                    this.CompileAgainstPublicly<WindowsSDK.WindowsSDK>(source);
                }
            });

All such calls that look for modules should happen outside of a patch, in the module's Init function.