Align Mu.so & Mu.not docs with design
Align Mu.so & Mu.not docs with design

The previous explanation of so() (double-negation) wasn't consistent
with implementation or mentioned in design docs. It's a common
JavaScript idiom but not relevant to Perl 6. This aligns with the docs
for prefix:<so> and :<not>, too. Ref #2693.

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