General improvements to Match documentation
General improvements to Match documentation

* mention that Match can be an in-progress match as well
* mention that method pos only makes sense for in-progress matches
* orig can return a non-string

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