Expands explanation on `is rw` trait
Expands explanation on `is rw` trait

With new examples, and a reference to `is rw` on the whole class. It
was probably better where it is, in classtut, rather than in the
typesystem, which didn't seem to have (at least now, 18 months after
OP) a place for it. This closes #1572

MasterDuke17/doc,tbrowder/doc,perl6/doc,MasterDuke17/doc,perl6/doc,perl6/doc,tbrowder/doc,tbrowder/doc,tbrowder/doc,MasterDuke17/doc,perl6/doc,MasterDuke17/doc,perl6/doc