Adds explanation on zero-width assertions
Adds explanation on zero-width assertions

Complements @moritz's addition in
895f932decdfd3f3b3b4b085ee10dbb3014d6f67 and closes #2023.

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