SASL: More informative help command for Set.
SASL: More informative help command for Set.

With #1338, the parameter username was made optional, but the help
was not changed to reflect this.

ref. https://github.com/znc/znc/pull/1338#issuecomment-270563035

Phansa/znc,kashike/znc,elyscape/znc,znc/znc,dgw/znc,GLolol/znc,GLolol/znc,Phansa/znc,kashike/znc,markusj/znc,jreese/znc,znc/znc,DarthGandalf/znc,evilnet/znc,Zarthus/znc,markusj/znc,kashike/znc,jreese/znc,jreese/znc,evilnet/znc,psychon/znc,elyscape/znc,evilnet/znc,kashike/znc,markusj/znc,znc/znc,Phansa/znc,BtbN/znc,BtbN/znc,Zarthus/znc,znc/znc,GLolol/znc,Zarthus/znc,DarthGandalf/znc,DarthGandalf/znc,GLolol/znc,DarthGandalf/znc,jreese/znc,psychon/znc,Phansa/znc,BtbN/znc,elyscape/znc,dgw/znc,DarthGandalf/znc,BtbN/znc,Zarthus/znc,markusj/znc,evilnet/znc,Phansa/znc,jreese/znc,psychon/znc,elyscape/znc,Phansa/znc,dgw/znc,Zarthus/znc,Zarthus/znc,kashike/znc,psychon/znc,elyscape/znc,dgw/znc,markusj/znc,dgw/znc,GLolol/znc,BtbN/znc,znc/znc,kashike/znc,jreese/znc,BtbN/znc,GLolol/znc,elyscape/znc,evilnet/znc,markusj/znc,psychon/znc,dgw/znc,evilnet/znc,znc/znc,psychon/znc,DarthGandalf/znc