[io grant] Document copy/move behaviour for same target/source
[io grant] Document copy/move behaviour for same target/source

RT#131242: https://rt.perl.org/Ticket/Display.html?id=131242
Rakudo fix: https://github.com/rakudo/rakudo/commit/08a8075f91
Tests: https://github.com/perl6/roast/commit/4fdb8504cd
gfldex/doc,gfldex/doc,perl6/doc,tbrowder/doc,perl6/doc,stmuk/doc,jonathanstowe/doc,antquinonez/doc,MasterDuke17/doc,perl6/doc,stmuk/doc,dmaestro/doc,tbrowder/doc,stmuk/doc,jonathanstowe/doc,dmaestro/doc,MasterDuke17/doc,antquinonez/doc,dmaestro/doc,antquinonez/doc,antquinonez/doc,gfldex/doc,antquinonez/doc,jonathanstowe/doc,jonathanstowe/doc,gfldex/doc,jonathanstowe/doc,MasterDuke17/doc,perl6/doc,MasterDuke17/doc,stmuk/doc,dmaestro/doc,stmuk/doc,gfldex/doc,perl6/doc,tbrowder/doc,tbrowder/doc,dmaestro/doc