Need to update docs re: fullPath change

For backwards compatibility, FileTransfer can take device-absolute file paths as well as file URLs.

Previously, there were two methods that would get you a file path from a FileEntry: Entry.toURL() and Entry.fullPath would both expose the absolute path. With the new version (1.0.0) of File, fullPath now properly represents the path relative to the filesystem root, rather than the device root.

We should update the docs to say that if you were previously using entry.fullPath to obtain filesystem paths, you should switch to entry.toURL() and use URLs instead.