feat: Expand section on Provision Watchers (#598)

docs_src/microservices/device/Ch-DeviceServices.md

@@ -87,7 +87,11 @@ In static device configuration (also known as static provisioning) the device se
 
 In dynamic discovery (also known as automatic provisioning), a device service is given some general information about where to look and general parameters for a device (or devices).  For example, the device service may be given a range of BLE address space and told to look for devices of a certain nature in this range.  However, the device service does not know that the device is physically there – and the device may not be there at start up.  It must continually scan during its operations (typically on some sort of schedule) for new devices within the guides of the location and device parameters provided by configuration. 
 
-Not all device services support dynamic discovery.  If it does support dynamic discovery, the configuration about what and where to look (in other words, where to scan) for new devices is specified by a provision watcher.  A provision watcher is created via a call to the [core metadata provision watcher API](https://app.swaggerhub.com/apis-docs/EdgeXFoundry1/core-metadata/2.0.0#/default/post_provisionwatcher) (and is stored in the metadata database).  In addition to providing details about what devices to look for during a scan, a provision watcher may also contain “blocking” indicators, which define parameters about devices that are not to be automatically provisioned.  This allows the scope of a device scan to be narrowed or allow specific devices to be avoided.  
+Not all device services support dynamic discovery.  If it does support dynamic discovery, the configuration about what and where to look (in other words, where to scan) for new devices is specified by a provision watcher.  A provision watcher is created via a call to the [core metadata provision watcher API](https://app.swaggerhub.com/apis-docs/EdgeXFoundry1/core-metadata/2.0.0#/default/post_provisionwatcher) (and is stored in the metadata database).
+
+A Provision Watcher is a filter which is applied to any new devices found when a device service scans for devices. It contains a set of ProtocolProperty names and values, these values may be regular expressions. If a new device is to be added, each of these must match the corresponding properties of the new device. Furthermore, a provision watcher may also contain “blocking” identifiers, if any of these match the properties of the new device (note that matching here is *not* regex-based), the device will not be automatically provisioned. This allows the scope of a device scan to be narrowed or allow specific devices to be avoided.  
+
+More than one Provision Watcher may be provided for a device service, and discovered devices are added if they match with any one of them. In addition to the filtering criteria, a Provision Watcher includes specification of various properties to be associated with the new device which matches it: these are the Profile name, the initial AdminState, and optionally any AutoEvents to be applied.
 
 ## Admin State