Add recommendation to use raw_attribute with ipynb.

See #5354.

MANUAL.txt

@@ -5036,6 +5036,18 @@ hello
 ::::::
 ````
 
+If you include raw HTML or TeX in an output cell, use the
+[raw attribute][Extension: `fenced_attribute`], as shown
+in the last cell of the example above.  Although pandoc can
+process "bare" raw HTML and TeX, the result is often
+interspersed raw elements and normal textual elements, and
+in an output cell pandoc expects a single, connected raw
+block.  To avoid using raw HTML or TeX except when
+marked explicitly using raw attributes, we recommend
+specifying the extensions `-raw_html-raw_tex+raw_attribute` when
+translating between Markdown and ipynb notebooks.
+
+
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