Update combat.as

Fixing kitsuneIllusion (line 5417):
* Most importantly: Fixed speed debuff abusable glitch (line 5449) and simplified the formula (would previously cap to -20 only if less than -50, now only decreases by 20 if 0 or more)
* Success rate roll simplified on line 5446 for slightly faster execution and better readability (substracted 1 from both sides) - mathematically speaking, it's still the same.
* Might want to check lustVuln on line 5450. Is Illusion really supposed to increase lust vulnerability by 10% *only* if target's lust vulnerability is 110% or more?
* Added a few comments.