Provide safer command to delete sharding jobs

This ensures that people who are blindly copy-pasting commands
don't accidentally delete running reshard jobs.

Signed-off-by: Rob Thomas <xrobau@linux.com>
diff --git a/src/cluster/sharding.rst b/src/cluster/sharding.rst
index a3075dc..01408d4 100644
--- a/src/cluster/sharding.rst
+++ b/src/cluster/sharding.rst
@@ -717,13 +717,12 @@
 section <config/reshard>` for the default value of ``max_jobs`` parameter and
 how to adjust if needed.
 
-For example, if the jobs have completed, to remove all the jobs run:
+For example, to remove all the completed jobs run:
 
 .. code-block:: bash
 
-    $ curl -s $COUCH_URL:5984/_reshard/jobs | jq -r '.jobs[].id' |\
-      while read -r jobid; do\
-          curl -s -XDELETE $COUCH_URL:5984/_reshard/jobs/$jobid\
+    $ for jobid in $(curl -s $COUCH_URL:5984/_reshard/jobs | jq -r '.jobs[] | select (.job_state=="completed") | .id'); do \
+          curl -s -XDELETE $COUCH_URL:5984/_reshard/jobs/$jobid \
       done
 
 Then it's a good idea to see what the db shard map looks like.