Document NEGQ instruction

[dansanderson]

As per this core issue tracking plans for new Q instructions, the new instruction NEGQ (assembled as neg neg neg, no argument) has been implemented and was released in v0.96. Confirm, test, and document this instruction.

It's not yet clear how much of the "wishlist" we intend to implement, so just document NEGQ for now.

[lgblgblgb]

Uh, even NEGQ was new for me, that it's already implemented ... I mean in the perspective of emulation, it's important to know if a new CPU opcode is out there already ;) If anybody knows about new opcodes already in the core, please tell me. Thanks.