Add example to extension RDoc in query extension
Add AssociationReflection#offset private method to DRY up some code
Add :from_self_alias entry to eager_grapher hash

This is to stop the default eager_graphers from looking into the
eager_graph metadata in the dataset.  I've decided not to document
this, as the use case for it is very rare.
Use an eager limit strategy by default for *_one associations with orders

If an order is specified for a *_one association, then the association
is probably not a true one-to-one association, and therefore could
probably benefit from an eager limit strategy.

Previously, this was only done automatically for *_one associations
if they had offsets.

With this change, the distinct on eager limit strategy can be used
by default.