DOCSP-20035 Adds info about unsupported Avro types (#212)

* DOCSP-20035 Adds info about unsupported Avro types

* Warning should be above the important callout, if we are going to keep said warning

* Adds workarounds for some unsupported types

* formatting, phrasing, adds more details

* Fix formatting

* tech review edits

Author: mongodbchez

source/introduction/data-formats.txt

@@ -150,6 +150,16 @@ specification of the following groups of data types:
 - `Complex Types <https://avro.apache.org/docs/current/spec.html#schema_complex>`__
 - `Logical Types <https://avro.apache.org/docs/current/spec.html#Logical+Types>`__
 
+.. warning:: Unsupported Avro Types
+
+   {+mkc+} does not support the following Avro types:   
+
+   - ``enum`` types. Use ``string`` instead.
+   - ``fixed`` types. Use ``bytes`` instead.
+   - ``null`` as a primitive type. However, ``null`` as an element in a ``union`` is supported.
+   - ``union`` types with more than 2 elements.
+   - ``union`` types with more than one ``null`` element.
+
 .. important:: Sink Connectors and Logical Types
 
    {+mkc+} sink connectors support all Avro schema primitive and complex types,