Documentation/security-bugs: Clarify treatment of embargoed information

The Linux kernel security team has been accused of rejecting the idea of
security embargoes. This is incorrect, and could dissuade people from
reporting security issues to us under the false assumption that the
issue would leak prematurely.

Clarify the handling of embargoed information in our process
documentation.

Co-developed-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Kees Cook <keescook@chromium.org>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>