Clarify that in unsupported open modes…
Clarify that in unsupported open modes…

…unsupportedness applies to reads/writes too, if the
handle managed to get opened in such a mode.

Related propspec: https://github.com/perl6/roast/commit/37aa7c3d7f
Related ticket:   R#1614 https://github.com/rakudo/rakudo/issues/1614

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