Docs for basePath need improvement

[Jakub Ś]

Based on #10187.

The only way to change base path is using GLOBAL variable - CKEDITOR_BASEPATH. After setting this value, CKEditor.basePath == CKEDITOR_BASEPATH and CKEDITOR.plugins.basePath == CKEDITOR_BASEPATH +'/plugins'.
Properties basePath and plugins.baePath are read-only and changing them will not have any result. If you look at the ticket #10187 you will see that not everyone knows that.

As discussed with @fredck I propose:

1. Marking them in docs as read-only values
2. Changing text for plugins.basePath from "The base directory containing all resources." to something like "Path to directory containing all plugins". First one may suggest that this is in fact path to CKEditor directory.