error: Add error_vprepend() in comment of ERRP_GUARD() rules

error: Add error_vprepend() in comment of ERRP_GUARD() rules

The error_vprepend() should use ERRP_GUARD() just as the documentation
of ERRP_GUARD() says:

> It must be used when the function dereferences @errp or passes
> @errp to error_prepend(), error_vprepend(), or error_append_hint().

Considering that error_vprepend() is also an API provided in error.h,
it is necessary to add it to the description of the rules for using
ERRP_GUARD().

Cc: Markus Armbruster <armbru@redhat.com>
Cc: Michael Roth <michael.roth@amd.com>
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240311033822.3142585-2-zhao1.liu@linux.intel.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>