Update --update-parallelism docs

Update documentation to account for the changes in #24952.

docs/swarm/swarm-tutorial/rolling-update.md doesn't need any changes,
but the CLI reference pages should show the current help text.
drain-node.md no longer needs to specify --update-parallelism 1 in its
example.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>

docs/reference/commandline/service_create.md

@@ -40,7 +40,7 @@ Options:
       --restart-window value         Window used to evaluate the restart policy (default none)
       --stop-grace-period value      Time to wait before force killing a container (default none)
       --update-delay duration        Delay between updates
-      --update-parallelism uint      Maximum number of tasks updated simultaneously
+      --update-parallelism uint      Maximum number of tasks updated simultaneously (0 to update all at once) (default 1)
   -u, --user string                  Username or UID
       --with-registry-auth           Send registry authentication details to Swarm agents
   -w, --workdir string               Working directory inside the container

docs/reference/commandline/service_update.md

@@ -47,7 +47,7 @@ Options:
       --restart-window value         Window used to evaluate the restart policy (default none)
       --stop-grace-period value      Time to wait before force killing a container (default none)
       --update-delay duration        Delay between updates
-      --update-parallelism uint      Maximum number of tasks updated simultaneously
+      --update-parallelism uint      Maximum number of tasks updated simultaneously (0 to update all at once) (default 1)
   -u, --user string                  Username or UID
       --with-registry-auth           Send registry authentication details to Swarm agents
   -w, --workdir string               Working directory inside the container