Adds a trap on combining WhateverCode and SmartMatch
Adds a trap on combining WhateverCode and SmartMatch

At the same time inaugurates a section for Smartmatch. Closes #1603 by using the same examples and explanations used there.

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