"Valkey is single-threaded" not good words

[zuiderkwast]

IO threading is barely mentioned in the docs. Using a simple grep for "thread", I find that wherever threads are mentioned, it is wrong. We seem to have missed updating the docs while improving I/O threading.

Now, I think we should at least correct the parts that are plain wrong. @uriyage, @ranshid do you want to help with this?

I found the following problems using a simple git grep thread in this repo. (It's old. We've just replaced Redis with Valkey.)

benchmark.md (fixed in #222):

Valkey is, mostly, a single-threaded server from the POV of commands execution (actually modern versions of Valkey use threads for different things). It is not designed to
benefit from multiple CPU cores. People are supposed to launch several Valkey instances to scale out on several cores if needed. It is not really fair to compare one single Valkey instance to a multi-threaded data store.

encryption.md (fixed in #222):

I/O threading is currently not supported with TLS.

faq.md (fixed in #197):

As of version 4.0, Valkey has started implementing threaded actions. For now this is limited to deleting objects in the background and blocking commands implemented via Valkey modules. For subsequent releases, the plan is to make Valkey more and more threaded.

latency-monitor.md:

Finally, Valkey is single threaded. This is usually an advantage

I think we should formulate this differently. It's not single threaded, but the commands are executed sequentially by a single thread. We shouldn't just state that it's single threaded.

latency.md (fixed in #222):

Single threaded nature of Valkey

[zuiderkwast]

@ranshid fyi

[uriyage]

Thanks @zuiderkwast , I will open a PR for it.