docs: threat-model: don't limit root capabilities to CAP_SYS_ADMIN

docs: threat-model: don't limit root capabilities to CAP_SYS_ADMIN

The threat-model document says that only users with CAP_SYS_ADMIN can carry
out a number of admin-level tasks, but there are numerous capabilities that
can confer that sort of power.  Generalize the text slightly to make it
clear that CAP_SYS_ADMIN is not the only all-powerful capability.

Acked-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>