[lldb] Correct documentation of LLDB_TEST_USER_ARGS (#89042)

https://discourse.llvm.org/t/running-lldb-in-a-container/76801/4 found
that the obvious way to use this variable doesn't work, despite what our
docs and examples say.

Perhaps in the past it did but now you need to use ";" as a separator to
make sure the final command line works properly.

For example "-A foo" becomes "-A foo" when python goes to run the runner
script. The script sees this as one command line element, not two. What
you actually want is "-A;foo" which we convert to "-A" "foo" which the
script sees as one option and one value for that option.

The "Script:" printout from dotest is misleading here because it does `"
".join(cmd)` so it looks like it's ok but in fact it's not.

I'm not changing that format though because printing the command as a
Python list is not useful outside of this specific situation.