Fix the "location" keyword classification to refer to schemas only

[handrews]

The current "location" behavior was kind of a dumping ground for keywords that didn't have an immediate action associated with them, namely $defs and $comment.

However, schema location behavior is critically important for proper detection of embedded resources and fragment declarations. As a behavioral marker for extension keywords, it informs an implementation where to look for these things. Regardless of what else we may or may not do with the current keyword classification section, we should fix this to be a more useful category. Agreeing to do so does not preclude a more substantial change in this area later, once other discussions have resolved.

$comment can be described as having unique behavior. If we eventually adopt a more elaborate keyword behaviors approach, it will fit into such a thing, but I don't want to worry about that much more complex topic right now. It's not clear that we'll act on that idea at all so it shouldn't block this simpler fix.