Adds documentation for void type
Adds documentation for void type

With some comments on how to use it, mainly in the context of native
calls. Which should be obvious, since they are native types. Closes #2130

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