Document Diablo's bugs

Currently documented bugs: [The DSF Buglist for Diablo v1.09 (Lurker Lounge)](http://www.lurkerlounge.com/forums/thread-12422.html)

The goal of this issue is to document any *new* bugs we come across while examining the code that are ***native*** to the **original** game. This will help us fix them later on when we make mods/ports.
# New bugs discovered (last update 06/24/18)
- When casting a town portal in a quest level, leaving, and then going down into the dungon, the town portal will be placed in the dungeon. For example, `Chamber of Bone` is quest level 2, so the portal would appear on dungeon level 2 at the same X/Y coordinate.
- Diablo is supposed to be immune to the squelch radius, however, the game checks the AI for Diablo's monster type, thus always failing. `monster.cpp -> ProcessMonsters`