Backport DOCSP-11917 to v6.0 (#5109)

* (DOCSP-11917) Advises not to set option to true for FCV 4.2 or earlier.

* Edits per copy review.

Author: erabil-mdb

source/reference/mongodb-agent-settings.txt

@@ -537,6 +537,9 @@ Externally Sourced Configuration Settings
    If you set this option to ``true``, the {+mdbagent+} doesn't write
    the :setting:`mmsConfigBackup` file.
 
+   Don't set this option to ``true`` if your MongoDB databases are running
+   :manual:`FCV </reference/command/setFeatureCompatibilityVersion>` 4.2 or earlier.
+
    .. warning::
 
       Setting this option to ``true`` impacts the availability of your