Check rendered docs, not raw pod
Check rendered docs, not raw pod

* respect TEST_THREADS
* ignore tables (requires a slightly recent rakudo)
* ignore some single-char non-letters, e.g.: / < >

(Whoops; part of this commit went in with b895a847)

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