man: ClientOnionAuthDir can't be reloaded with Sandbox 1

By adding a file to the ClientOnionAuthDir and sending a HUP signal, tor would
load the new file and use it. However, that doesn't work with the Sandbox
since post initilization, nothing can be changed.

Document in the manpage that limitation within the Sandbox description.

Closes #28128

Signed-off-by: David Goulet <dgoulet@torproject.org>