Fixes #10902 by adding a mention of the JSON extraction in the docume…

…ntation.

Co-authored-by: Jim Fehrle <jim.fehrle@gmail.com>
(cherry picked from commit 87d6d18)

doc/sphinx/addendum/extraction.rst

@@ -99,12 +99,15 @@ Extraction Options
 Setting the target language
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-.. cmd:: Extraction Language {| OCaml | Haskell | Scheme }
+.. cmd:: Extraction Language {| OCaml | Haskell | Scheme | JSON }
    :name: Extraction Language
 
    The ability to fix target language is the first and more important
    of the extraction options. Default is ``OCaml``.
 
+   The JSON output is mostly for development or debugging:
+   it contains the raw ML term produced as an intermediary target.
+
 
 Inlining and optimizations
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~