Add more detail to help text for reclaim_instance_interval option

The reclaim_instance_interval config option is read from both the
API and compute nodes so it's important for people using this option
to understand that they need to set it in both places, because otherwise
you could soft delete an instance from the API but it will not get
reclaimed (hard deleted) unless the compute host it's running on also
has that same configuration set.

This also fixes the point about how to get an instance out of the
soft deleted state before it's reclaimed, which is via the 'restore'
action, not a 'recover' action which doesn't exist in the REST API.

Change-Id: I0aecaea7b76a57ed3b409608800eeaea151451d0
Partial-Bug: #1650328

nova/conf/compute.py

@@ -428,14 +428,19 @@
 Interval for reclaiming deleted instances.
 
 A value greater than 0 will enable SOFT_DELETE of instances.
-This option decides whether the server to be deleted will still be in
+This option decides whether the server to be deleted will be put into
 the SOFT_DELETED state. If this value is greater than 0, the deleted
 server will not be deleted immediately, instead it will be put into
 a queue until it's too old (deleted time greater than the value of
 reclaim_instance_interval). The server can be recovered from the
-delete queue by using recover action. If the deleted server remains
+delete queue by using the restore action. If the deleted server remains
 longer than the value of reclaim_instance_interval, it will be
-deleted by compute service automatically.
+deleted by a periodic task in the compute service automatically.
+
+Note that this option is read from both the API and compute nodes, and
+must be set globally otherwise servers could be put into a soft deleted
+state in the API and never actually reclaimed (deleted) on the compute
+node.
 
 Possible values: