doc tweaks

docs/releng.adoc

@@ -33,6 +33,10 @@ is always functional. You can also run the process locally with
 
 == Process overview
 
+`cargo xtask releng` performs all of these steps in parallel (with
+the temporary exception of artifact downloads handled by
+`tools/install_builder_prerequisites.sh`):
+
 . `tools/install_builder_prerequisites.sh` downloads several artifacts
   (via the `tools/ci_*` scripts) that are necessary to build Omicron;
   many of these are ultimately packaged by `omicron-package`. These
@@ -49,9 +53,11 @@ is always functional. You can also run the process locally with
   packages them into a series of illumos zones and tarballs. (It can
   also manage installation and uninstallation of these zones; see
   how-to-run.adoc.)
-. Some of the packaged artifacts are installed directly in the OS
-  images; `cargo xtask releng` unpacks these into a temporary directory
-  that is overlaid onto the OS image in the next step.
+. Some of the illumos zones are distributed with the OS images (because
+  they are reliant on OS-specific APIs), and some are distributed
+  separately. `cargo xtask releng` unpacks the zones for the OS image
+  into a temporary directory that is overlaid onto the OS image in the
+  next step.
 . `helios-build` from the https://github.com/oxidecomputer/helios[Helios]
   repository then builds two images: the *host* image, which is used
   during normal operation, and the *trampoline* (or *recovery*) image,
@@ -63,10 +69,6 @@ is always functional. You can also run the process locally with
   can be uploaded to Wicket to perform an upgrade of the rack while the
   control plane is not running.
 
-`cargo xtask releng` performs all of these steps in parallel (with
-the temporary exception of artifact downloads handled by
-`tools/install_builder_prerequisites.sh`.)
-
 == Beyond `cargo xtask releng`
 
 Currently we use TUF repos generated in CI (by `cargo xtask releng`)