Adds examples using backreferences in regexes
Adds examples using backreferences in regexes

This closes #1545. Since this part is going to probably be moved dur
to #2890, it will move with them; this seemed the most reasonable part
to document these back references, since it's where they are actually used.

perl6/doc,MasterDuke17/doc,perl6/doc,MasterDuke17/doc,perl6/doc,tbrowder/doc,perl6/doc,tbrowder/doc,MasterDuke17/doc,perl6/doc,MasterDuke17/doc,tbrowder/doc,tbrowder/doc