Improve documentation of sleep-timer.
Improve documentation of sleep-timer.

Close #1795

Source code from Date shipped with moar2018.01.

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