Remove needless prose in .flat
Remove needless prose in .flat

- Flat returns a Seq with an iterator fed by original stuff, so it solving any
    halting problems is irrelevant.
- Flat used to have a bug where it was failing to propagate .is-lazy in certain
   cases, causing full reification, which might be what the original author hit
   upon, to surmise there are any issues with flattening infinite things.
- Fixes #1202
MasterDuke17/doc,tbrowder/doc,antquinonez/doc,perl6/doc,MasterDuke17/doc,dmaestro/doc,stmuk/doc,tbrowder/doc,jonathanstowe/doc,gfldex/doc,antquinonez/doc,jonathanstowe/doc,dmaestro/doc,stmuk/doc,dmaestro/doc,perl6/doc,jonathanstowe/doc,antquinonez/doc,gfldex/doc,perl6/doc,antquinonez/doc,MasterDuke17/doc,perl6/doc,tbrowder/doc,gfldex/doc,stmuk/doc,tbrowder/doc,antquinonez/doc,perl6/doc,jonathanstowe/doc,dmaestro/doc,stmuk/doc,jonathanstowe/doc,gfldex/doc,gfldex/doc,MasterDuke17/doc,dmaestro/doc,stmuk/doc