Show same show-doc results for ::Klass and Klass

Fixes issue #960. `show-doc ::Array` wasn't showing the same results as `show-doc Array` because of how top-level constants are keyed in the YARD registry. 

This takes care of showing the correct results for `show-doc ::Klass`, unless anyone can think of any case where we wouldn't want the same results for `show-doc ::Klass` and `show-doc Klass`?