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

See issue #1795

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