Document `--repl-mode` cmd opt
Document `--repl-mode` cmd opt

Impl: https://github.com/rakudo/rakudo/commit/93e599dbf2
      https://github.com/perl6/nqp/commit/943f7f757c

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