doc: add info about JSON output

doc/dynare.texi

@@ -839,8 +839,34 @@ derivatives with respect to the parameters that are calculated by the
 preprocessor. @code{0} means no derivatives, @code{1} means first derivatives,
 and @code{2} means second derivatives. Default: @code{2}
 
-@item nowarn
-Suppresses all warnings.
+@anchor{json}
+@item json=parse|transform|compute
+Causes the preprocessor to output a version of the @file{.mod} file in JSON
+format. If @code{parse} is passed, then the output will be written after the
+parsing step in a file called @file{@var{FILENAME}.mod}. If @code{transform} is
+passed, the JSON output of the transformed model (maximum lead of 1, minimum
+lag of -1, expectation operators substituted, etc.) will be written after the
+transform pass in a file called @file{@var{FILENAME}.mod}. And if
+@code{compute} is passed, the output is written after the computing pass. Here,
+the transformed model is written to @file{@var{FILENAME}.mod}, the original
+model is written to @file{@var{FILENAME}_original.mod}, and the dynamic and
+static files are written to @file{@var{FILENAME}_dynamic.mod} and
+@file{@var{FILENAME}_static.mod}.
+
+@item jsonstdout
+Instead of writing output requested by @ref{json} to files, write to standard
+out.
+
+@item onlyjson
+Quit processing once the output requested by @ref{json} has been written.
+
+@item jsonprintderivdetail
+In addition to the output provided when @code{compute} is passed to @ref{json},
+create two files, @file{@var{FILENAME}_dynamic_details.mod} and
+@file{@var{FILENAME}_static_details.mod}. The difference between these files
+and their @file{_dynamic.mod} and @file{_static.mod} counterparts is that these
+files provide information about which variable(s) and equation the derivative
+corresponds to.
 
 @item warn_uninit
 Display a warning for each variable or parameter which is not