Refer to docs in case of duplicate assets

[sh-dave]

from irc:

• dave: i just updated the wiki section for kha's asset management to include the "advanced" options. come to think of it, maybe they should be the default, as that's what people would expect instead flattened directories?
• RobertMobile: Not sure. I like to put images in an Image subdir and don't like to have Assets.images.Images_whatever.
• RobertMobile: But we could detect duplicates and then refer to the docs.