DOCSP-38696 Add faq regarding fatal errors (#305)

* DOCSP-38696 add faq

* DOCSP-38696 jason feedback

Author: jocelyn-mendez1

source/faq.txt

@@ -136,6 +136,14 @@ These logs do not indicate that ``mongosync`` is failing or corrupting
 data. If a fatal error occurs, ``mongosync`` stops the sync and writes a
 fatal log entry. 
 
+What should I do if mongosync returns a fatal error?
+----------------------------------------------------
+
+A fatal error indicates a problem that must be corrected and requires the 
+migration to be restarted. After addressing the error, delete all migrated 
+data on the destination cluster, including the ``mongosync_reserved_for_internal_use`` 
+database. Then, relaunch ``mongosync`` and start a new migration.
+
 Does ``mongosync`` Support TTL Indexes?
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