Clarify :set! documentation.
Clarify :set! documentation.

Closes issue #519.

--HG--
extra : rebase_source : 18f6188af12ec41c4c7ea0a1e8789422b13717ff

invl/dactyl,airodactyl/dactyl,Kjwon15/dactyl,invl/dactyl,invl/dactyl,5digits/dactyl,Kjwon15/dactyl,Kjwon15/dactyl,madand/pentadactyl-pm,moonshadow/dactyl,invl/dactyl,madand/pentadactyl-pm,nathyong/dactyl,5digits/dactyl,madand/pentadactyl-pm,5digits/dactyl,moonshadow/dactyl,Kjwon15/dactyl,moonshadow/dactyl,madand/pentadactyl-pm,5digits/dactyl,airodactyl/dactyl,5digits/dactyl,madand/pentadactyl-pm,madand/pentadactyl-pm,nathyong/dactyl,invl/dactyl,nathyong/dactyl,moonshadow/dactyl,airodactyl/dactyl,nathyong/dactyl,Kjwon15/dactyl,nathyong/dactyl,5digits/dactyl,invl/dactyl,airodactyl/dactyl,airodactyl/dactyl,moonshadow/dactyl,moonshadow/dactyl,Kjwon15/dactyl,airodactyl/dactyl,nathyong/dactyl