documentation: improve the unsafe-yes documentation for confirm-level

[hannesm]

The documentation is rather sparse (opam 2.1.5 here):

       --confirm-level=LEVEL
           Confirmation level, LEVEL must be one of `ask', `no', `yes' or
           `unsafe-yes'. Can be specified more than once. If --yes or --no are
           also given, the value of the last --confirm-level is taken into
           account. This is equivalent to setting  $OPAMCONFIRMLEVEL`.

And then, --yes and --no are documented more clearly:

       --no
           Answer no to all opam yes/no questions without prompting. See also
           --confirm-level. This is equivalent to setting $OPAMNO to "true".

       -y, --yes
           Answer yes to all opam yes/no questions without prompting. See also
           --confirm-level. This is equivalent to setting $OPAMYES to "true".

But the unsafe-yes is not described much. What is the "unsafe" part in it? This issue has been opened due to investigation by @ben-grande in mirage/qubes-mirage-firewall#187