Allow OUTER nesting and document how it works

[beikov]

OUTER allows to refer to properties of the outer query without knowing the aliases of the roots. Allow nesting of OUTERs and document the allowed usage and behavior. In addition to that, also allow the use of OUTER without arguments to refer to the alias itself which might be nice to do correlated subqueries. Document that it is allowed in Chapter 18. JPQL functions

[beikov]

Also implement proper fromClause propagation throughout the process. Currently the wrong clause dependency is registered on a newly generated join node.