DOCS-3969: add clarification about csv format

source/includes/options-mongoimport.yaml

@@ -209,6 +209,13 @@ args: <json|csv|tsv>
 description: |
   Specifies the file type to import. The default format is :term:`JSON`,
   but it's possible to import :term:`csv` and :term:`tsv` files.
+
+  The ``csv`` parser accepts that data that complies with RFC
+  :rfc:`4180`. As a result, backslashes are *not* a valid escape
+  character. If you use double-quotes to enclose fields in the CSV
+  data, you must escape internal double-quote marks by prepending
+  another double-quote.
+
 optional: true
 ---
 program: mongoimport