Edit the help messages for "new" and "edit"

Edit the help messages for "new" and "edit"

The old help messages were confusing. It wasn't always clear that
"[puzzle]" meant an optional argument of literally the word "puzzle"
rather than the name of a particular puzzle to edit.

Instead, we encourage people to use these commands without arguments
(whenever possible, such as "/edit" for hunt or puzzle and "/new" for
puzzle) and to use clear literal arguments when necessary (such as
"/new hunt" from any channel or "/edit hunt" from within a puzzle
channel).