DOCS-4703: document database size limit

Author: i80and

source/reference/limits.txt

@@ -132,6 +132,13 @@ Data
    creating a capped collection, there is no limit on the number of
    documents.
 
+.. _limit-database-size:
+.. limit:: Database Size
+
+   The MMAPv1 storage engine limits each database to no more than 16000 data
+   files. This means that a single MMAPv1 database has a maximum size of 32TB.
+   Setting the :setting:`storage.mmapv1.smallFiles` option reduces this limit to 8TB.
+
 .. _limit-data-size:
 .. limit:: Data Size