DOCS-8986 clarify compact with MMAPv1 and WiredTiger

Author: kevinadi

source/reference/command/compact.txt

@@ -207,9 +207,10 @@ You cannot issue :dbcommand:`compact` against a :binary:`~bin.mongos` instance.
 Capped Collections
 ~~~~~~~~~~~~~~~~~~
 
-It is not possible or necessary to compact :term:`capped collections <capped
-collection>` because they lack padding and their documents cannot grow.
-As a result, they cannot become fragmented.
+On :ref:`MMAPv1 <storage-mmapv1>`, it is not possible to compact :term:`capped collections <capped
+collection>`.
+
+On :ref:`WiredTiger <storage-wiredtiger>`, the :dbcommand:`compact` command will attempt to compact the collection.
 
 Index Building
 ~~~~~~~~~~~~~~