Fixup roundrobin docs
Fixup roundrobin docs

- It ain't a method, but a sub
    Closes https://github.com/rakudo/rakudo/issues/1447
- Tweak description: lists *can* be of equal length if the number of
    items is the same
jonathanstowe/doc,stmuk/doc,MasterDuke17/doc,perl6/doc,dmaestro/doc,perl6/doc,jonathanstowe/doc,jonathanstowe/doc,antquinonez/doc,tbrowder/doc,antquinonez/doc,stmuk/doc,antquinonez/doc,dmaestro/doc,gfldex/doc,MasterDuke17/doc,perl6/doc,tbrowder/doc,gfldex/doc,jonathanstowe/doc,gfldex/doc,antquinonez/doc,perl6/doc,perl6/doc,stmuk/doc,stmuk/doc,dmaestro/doc,tbrowder/doc,MasterDuke17/doc,gfldex/doc,antquinonez/doc,MasterDuke17/doc,dmaestro/doc,tbrowder/doc,dmaestro/doc,stmuk/doc,gfldex/doc,jonathanstowe/doc