[m2445] Fix use of config values related to quest-status and level
This fixes use of negative value in config values Quests.LowLevelHideDiff and Quests.HighLevelHideDiff

Also add some documentation around the related code.

Backport of cmangos/mangos-classic@e070bff792

Signed-off-by: Daniel S. Reichenbach <daniel@kogitoapp.com>