Fix up info on Type Constraints
Fix up info on Type Constraints

Closes https://github.com/perl6/doc/issues/1870 D#1870
Closes https://github.com/perl6/doc/pull/1871   D#1871
Based on https://irclog.perlgeek.de/perl6/2018-03-26#i_15968371

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