Document DEBUG subcommands

[stockholmux]

The docs have a container for the DEBUG command but the subcommands remain undocumented.

There are very useful subcommands here, many of which have been around for a decade without proper documentation. It's obvious that these commands are potentially highly dangerous and blocked by default, but documenting them for normal users has real utility.

As an example, DEBUG POPULATE is super useful to a lot of folks testing out an Valkey, I don't think it's reasonable to keep this information from users.

[madolson]

We didn't document them in Redis since that implies we won't break them in the future without deprecating them. We don't want anyone to take a dependency. We just need to make that behavior clear. I think we should also only selectively document them.