Clarify explanation of Verity= option

This clarifies that the "auto" value for the verity option
only really makes sense for extension images.

mkosi/resources/man/mkosi.1.md

@@ -1136,9 +1136,9 @@ boolean argument: either `1`, `yes`, or `true` to enable, or `0`, `no`,
  certificate must be present and the build will fail if we don't
  detect any verity partitions in the disk image produced by
  systemd-repart. If disabled, verity partitions will be excluded from
- disk images produced by systemd-repart even if the partition
- definitions contain verity partitions. If set to `auto`, the verity
- key and certificate will be passed to systemd-repart if available,
+ the extension images produced by systemd-repart. If set to `auto` and
+ a verity key and certificate are present, mkosi will pass them to systemd-repart
+ and expects the generated disk image to contain verity partitions,
  but the build won't fail if no verity partitions are found in the
  disk image produced by systemd-repart.