Documentation/srso: Document IBPB aspect and fix formatting

Add a note about the dependency of the User->User mitigation on the
previous Spectre v2 IBPB selection.

Make the layout moar pretty.

Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20230809102700.29449-4-bp@alien8.de