Merge pull request #1797 from bgrabar/glossary-item

Add an entry for "lock" to the glossary.

Author: Bob Grabar

source/reference/glossary.txt

@@ -388,6 +388,12 @@ Glossary
       LineString in MongoDB, see
       :ref:`geospatial-indexes-store-geojson`.
 
+   lock
+      MongoDB uses locks to ensure :doc:`concurrency </faq/concurrency>`.
+      MongoDB uses both :term:`read locks <read lock>` and :term:`write
+      locks <write lock>`. For more information, see
+      :ref:`faq-concurrency-locking`.
+
    LVM
       Logical volume manager. LVM is a program that abstracts disk
       images from physical devices and provides a number of raw disk