Document Failure.self
Document Failure.self

Rakudo impl: https://github.com/rakudo/rakudo/commit/0a100825dd
Tests: https://github.com/perl6/roast/commit/5bdb393ea9
stmuk/doc,dmaestro/doc,dmaestro/doc,perl6/doc,jonathanstowe/doc,antquinonez/doc,dmaestro/doc,jonathanstowe/doc,antquinonez/doc,tbrowder/doc,MasterDuke17/doc,antquinonez/doc,MasterDuke17/doc,MasterDuke17/doc,dmaestro/doc,stmuk/doc,tbrowder/doc,jonathanstowe/doc,antquinonez/doc,gfldex/doc,dmaestro/doc,antquinonez/doc,gfldex/doc,perl6/doc,stmuk/doc,stmuk/doc,jonathanstowe/doc,tbrowder/doc,perl6/doc,gfldex/doc,gfldex/doc,jonathanstowe/doc,tbrowder/doc,MasterDuke17/doc,stmuk/doc,gfldex/doc,perl6/doc,perl6/doc