DOCS-15440 dollar prefixed fields (#44)

* DOCS-15440 dollar prefixed fields

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Author: Dave Cuthbert

source/reference/limitations.txt

@@ -48,6 +48,9 @@ General Limitations
 - The MongoDB Shared Tier is not supported.
 - The :data:`system.js <<database>.system.js>` collection is not
   replicated.
+- Documents that have dollar (``$``) prefixed field names are not
+  supported. See :ref:`Field Names with Periods and Dollar Signs
+  <crud-concepts-dot-dollar-considerations>`.
 
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