Remove detritus from MidRatStr reference
Remove detritus from MidRatStr reference

When allomorphs were first documented in dcd83b, `MidRatStr` still
existed as a three-superclass allomorph. That's no longer the case, so
remove language referring to hypothetical three-class allomorphs.

But leave open the possibility so this passage isn't brittle against
future allomorphs.

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