Document illegal item names in Oak

From OAK-4857. Oak like Jackrabbit 2 has limits on valid node/property names in addition to the illegal chars from the JCR spec. This isn't documented yet.

Here is what I know so far:

• illegal node name if entire name is empty or . or ..
• no length limit (?)
• otherwise name can have all unicode chars except:
• JCR illegal chars / : [ ] | *
• Character.isWhitespace(), except for regular space u20 which is allowed, except first or last char