Cef.Initialize - Add more detail to exception through

Hopefully this reduces the support requests

Author: amaitland

CefSharp.Core/Cef.h

@@ -168,7 +168,11 @@ namespace CefSharp
             if (IsInitialized)
             {
                 // NOTE: Can only initialize Cef once, to make this explicitly clear throw exception on subsiquent attempts
-                throw gcnew Exception("Cef can only be initialized once. Use Cef.IsInitialized to guard against this exception.");
+                throw gcnew Exception("CEF can only be initialized once per process. This is a limitation of the underlying " + 
+					"CEF/Chromium framework. You can change many (not all) settings at runtime through RequestContext.SetPreference. " + 
+					"See https://github.com/cefsharp/CefSharp/wiki/General-Usage#request-context-browser-isolation " +
+					"Use Cef.IsInitialized to guard against this exception. If you are seeing this unexpectedly then you are likely " +
+					"calling Cef.Initialize after you've created an instance of ChromiumWebBrowser, it must be before the first instance is created.");
             }
 
             if (cefSettings->BrowserSubprocessPath == nullptr)