Improve C<flat> docs
Improve C<flat> docs

- use more detailed examples that show actual flattening
- include that Maps get flattened too, since they're Iterable
- Get rid of broken and convoluted `deepmap` example; it destroys Maps and Pairs
stmuk/doc,gfldex/doc,stmuk/doc,stmuk/doc,perl6/doc,antquinonez/doc,dmaestro/doc,gfldex/doc,stmuk/doc,MasterDuke17/doc,stmuk/doc,antquinonez/doc,antquinonez/doc,jonathanstowe/doc,gfldex/doc,dmaestro/doc,MasterDuke17/doc,antquinonez/doc,jonathanstowe/doc,MasterDuke17/doc,gfldex/doc,perl6/doc,jonathanstowe/doc,perl6/doc,tbrowder/doc,tbrowder/doc,MasterDuke17/doc,dmaestro/doc,antquinonez/doc,perl6/doc,tbrowder/doc,dmaestro/doc,dmaestro/doc,gfldex/doc,perl6/doc,jonathanstowe/doc,jonathanstowe/doc,tbrowder/doc