make it clearer how to set a specific Julia version for an environment

[bvdmitri]

I don't mind using 1.12 by default but some projects use Reactant/Enzyme and it works best under 1.11 so I want to use 1.11 for them by default from their folder

From a brief discussion in slack with @KristofferC it turned out it is already possible with

juliaup override set 1.11

but the help doesn't suggest that its possible and is very vague

→ juliaup override --help
Manage directory overrides

Usage: juliaup override <COMMAND>

Commands:
  status
  set
  unset
  help    Print this message or the help of the given subcommand(s)

Options:
  -h, --help  Print help

→ juliaup override set --help
Usage: juliaup override set [OPTIONS] <CHANNEL>

Arguments:
  <CHANNEL>

Options:
  -p, --path <PATH>
  -h, --help         Print help

In particular, the phrase "Manage directory overrides" doesn't really help