It is not trivial how detailed of a description you need to give about the system. It sounds a bit lazy, but if there is a good document explaining the basics, you could refer to that. Though that may increase the amount of people ignoring all of it.
I'd talk about CPU address space. Parts of it resolve to RAM, part of it resolves to registers galore, the rest resolves to cartridge. The cartridge may resolve its share of the addresses to external RAM or ROM.
PRG-RAM sounds bad/wrong. I don't think that adding that sentence makes it any better. PRG-RAM actually makes me think of the cartridge RAM. I don't know how unprofessional that makes me.
Don't be afraid of removing text (rewriting), if you want the guide to read well.
Keep in mind that a game does not use/access ROM addresses. They do not exist. A game will only use RAM addresses.
Related to earlier, I think this is bad/wrong. There are only CPU addresses (I'm not familiar with PPU). CPU opcodes (or opcodes with arguments?) go to the CPU and contain numbers interpreted as addresses in the CPU address space.
A debugger is mainly used to ...
I'd avoid "opinions" and merely present the debugger functionalities that are going to be presented. Or list them "all" and present some.
If address space is a difficult term to use, "memory" or "CPU memory" might ~work.
I'd have something like (not entirely sure when it stops actually, too lazy to think about instruction fetch cycles):
Read - Execution pauses when the current instruction loads from the given memory address.
Write - Execution pauses when the current instruction stores to the given memory address.
Execute- Execution pauses when the current instruction is in the given memory address.
It has been a while since I used a NES debugger. Is there a certain best version of FCEUX you refer to?
By the way, do you need that save state? Would a massive trace dump contain all the necessary information? Sometimes "keep clicking" is not very convenient.
It may be a bit difficult to give a generic guide to debugging. A set of tools that can be used in many ways. I guess many hackers here are interested in translations, if so, then this is an okay example. (it is a bit more complex than the basic ASM hacking I'm used to)