wmlscope: fix mistakes in help and options pointed out by Elvish Hunter

See EH's forum post:

http://forums.wesnoth.org/viewtopic.php?f=10&t=40397#p570438
add janebot to credits
wmllint: fix substitution that was missing a slash
wmllint: reorganize help into mode options and other options

The organization of Elvish Hunter's wmllint GUI tab suggested this change.