admin guide: use mgrctl on container host

[keichwa]

https://github.com/SUSE/spacewalk/issues/25390

[m-czernek]

+1, this looks good to me.

Btw I wouldn't change anything here, but it's interesting that we say on the container host, regenerate ....

In reality, mgrctl exec opens a terminal session and executes the command (in this case, spacecmd ...) inside of the container, so we actually regenerate the file in the container.

You're probably aware of the above, and I don't think we need to change anything here (probably too low-level information for the docs). But perhaps elsewhere, this distinction might be useful.