Extended the duckmap explanation
Extended the duckmap explanation

With the aid of [Brad
Gilbert](https://stackoverflow.com/questions/50201995/what-does-duckmap-really-do)
in StackOverflow and @timo in the IRC channel. Also linked to deepmap,
which is not linked back to avoid confusion. Closes #943.

MasterDuke17/doc,perl6/doc,stmuk/doc,perl6/doc,stmuk/doc,MasterDuke17/doc,stmuk/doc,perl6/doc,dmaestro/doc,dmaestro/doc,MasterDuke17/doc,stmuk/doc,jonathanstowe/doc,tbrowder/doc,perl6/doc,tbrowder/doc,jonathanstowe/doc,jonathanstowe/doc,tbrowder/doc,dmaestro/doc,stmuk/doc,jonathanstowe/doc,perl6/doc,dmaestro/doc,tbrowder/doc,MasterDuke17/doc,dmaestro/doc,jonathanstowe/doc