docs/system: standardize man page sections to --- with overline

Man pages in docs/system use file inclusion heavily.  Use headings with
overlines in the main files, so that the same included file work well
from both manuals and man pages.

This style of heading is a bit more heavy-weight, so it is not used by
the other man pages in interop/ and tools/.  If in the future they
are changed to use include files, for example to avoid having sections
named "synopsis" or "description", they can switch to --- with overline
as well.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>