qcacld-3.0: Add osif_sync high level documentation

While the concepts of osif_sync started out as a simple set of APIs, it
ultimately evolved into its own full-fledge code module. While the
per-function documenation is present, high-level documentation for the
osif_sync code module as a whole was over looked. Add a high-level
description of the purpose of osif_sync in osif_sync.h

Change-Id: I42cd28673b500c85ecf95de4b507b677804892ba
CRs-Fixed: 2421801

os_if/sync/inc/osif_sync.h

@@ -16,6 +16,30 @@
  * PERFORMANCE OF THIS SOFTWARE.
  */
 
+/**
+ * DOC: Operating System Interface, Synchronization (osif_sync)
+ *
+ * osif_sync is a collection of APIs for synchronizing and safely interacting
+ * with external processes/threads via various operating system interfaces. It
+ * relies heavily on the Driver Synchronization Core, which defines the
+ * synchronization primitives and behavior.
+ *
+ * While DSC is great at doing the required synchronization, it (by design) does
+ * not address the gap between receiving a callback and involing the appropriate
+ * DSC protections. For example, given an input net_device pointer from the
+ * kernel, how does one safely aquire the appropriate DSC context? osif_sync
+ * implements this logic via wrapping DSC APIs with a registration mechanism.
+ *
+ * For example, after the creation of a new dsc_vdev context, osif_sync allows
+ * it to be registered with a specific net_device pointer as a key. Future
+ * operations against this net_device can use this pointer to atomically lookup
+ * the DSC context, and start either a DSC transition or DSC operation. If this
+ * is successful, the operation continues and is guaranteed protected from major
+ * state changes. Otherwise, the operation attempt is rejected.
+ *
+ * See also: wlan_dsc.h
+ */
+
 #ifndef __OSIF_SYNC_H
 #define __OSIF_SYNC_H