docs(ci): clarify parallel manual matrix runs

.crow/weekly-rebuild-missing.yaml

@@ -4,9 +4,9 @@
 #   - cron:   each existing `weekly-rebuild-missing-<os>-<arch>` cron fires only
 #             its matching matrix rows (via the per-row `cron:` name filter),
 #             which is now all three shards of that slot.
-#   - manual: `weekly_rebuild_missing` dropdown, default "all" (matches the
-#             previous bare manual trigger that ran every os/arch); pick a
-#             single <os>-<arch> to run just one.
+#   - manual: pick a target from the `weekly_rebuild_missing` dropdown;
+#             "all" fans out every os/arch and shard as parallel matrix
+#             workflows, while a single <os>-<arch> runs its three shards.
 # Arch placement is handled by the group label (rpkgs-amd64, rpkgs-arm64).
 #
 # The shard picks up its own slice and re-derives what is still outstanding
@@ -21,7 +21,7 @@ variables:
   # .crow/, and a declared default is applied even when the run never passed
   # this variable, so the default must be a value that matches no matrix row.
   weekly_rebuild_missing:
-    description: "Manual run target: a specific <os>-<arch>, 'all' for every os/arch, or 'none' to run nothing."
+    description: "Manual run target: a specific <os>-<arch>, 'all' to run every os/arch in parallel, or 'none' to run nothing."
     options:
       - none
       - all