Add README for Lock library, fix issue with lock release, add a comment

Author: paveq

app/code/Magento/Cron/Observer/ProcessCronQueueObserver.php

@@ -227,6 +227,8 @@ public function execute(\Magento\Framework\Event\Observer $observer)
                 continue;
             }
 
+            // Note: we acquire the lock here instead of above, as it should be taken by standalone (child) process,
+            // not by the parent process.
             if (!$this->lockManager->setLock(self::LOCK_PREFIX . $groupId, self::LOCK_TIMEOUT)) {
                 $this->logger->warning(
                     sprintf(
@@ -277,7 +279,7 @@ public function execute(\Magento\Framework\Event\Observer $observer)
                 $schedule->save();
             }
 
-            $this->lockManager->releaseLock($groupId);
+            $this->lockManager->releaseLock(self::LOCK_PREFIX . $groupId);
         }
     }
 

lib/internal/Magento/Framework/Lock/README.md

@@ -0,0 +1,8 @@
+# Lock library
+
+Lock library provides mechanism to acquire Magento system-wide lock. Default implementation is based on MySQL locks, where any locks are automatically released on connection close.
+
+The library provides interface *LockManagerInterface* which provides following methods:
+* *setLock* - Sets a named lock
+* *releaseLock* - Releases a named lock
+* *isLocked* - Tests if a named lock exists