Added more documentation to Custom spells

More explanations for spell asterisk conventions and such.
Fixed Thunderfury Proc

Should no longer affect NPCs (and try to launch them into weird
places..)
Adjusted Spell Buff Durations

Up to 10 minutes from 5 minutes.
Fixed Windseeker "Has Thunderfury" event.

Should now properly allow players with thunderfury to 'rest' and receive
a nice HP buff.