DOCS-10720 Clarify 'file bytes available for reuse' in WT FAQ

Author: kevinadi

source/faq/storage.txt

@@ -84,6 +84,25 @@ intervals or condition:
 
 .. include:: /includes/extracts/wt-journal-frequency.rst
 
+.. _faq-wt-reclaim-disk-space:
+
+How do I reclaim disk space in WiredTiger?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The WiredTiger storage engine maintains lists of empty records in data
+files as it deletes documents. This space can be reused by WiredTiger,
+but will not be returned to the operating system unless under very
+specific circumstances.
+
+The amount of empty space available for reuse by WiredTiger is reflected
+in the output of :method:`db.collection.stats()` under the heading
+``wiredTiger.block-manager.file bytes available for reuse``.
+
+To allow the WiredTiger storage engine to release this empty space to the
+operating system, you can de-fragment your data file. This can be achieved
+using the :dbcommand:`compact` command. For more information on its behavior
+and other considerations, see :dbcommand:`compact`.
+
 MMAPv1 Storage Engine
 ---------------------