[CaR Grant] Document Rational.base-repeating with no args
[CaR Grant] Document Rational.base-repeating with no args

Spec: https://github.com/perl6/roast/commit/7263105372
Rakudo impl: since before the Christmas

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