Document Rational.floor/.ceiling
Document Rational.floor/.ceiling

They differ from Real's in that they can `fail`
POV for failing: https://github.com/rakudo/rakudo/commit/6d70346cbf
Propspec for failing: https://github.com/perl6/roast/commit/17761c2e48

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