docs(man): write about --output-format option

Based on the proposal from @k-takata in universal-ctags#2199.

man/ctags.1.rst.in

@@ -570,6 +570,7 @@ are not listed here. They are experimental or for debugging purpose.
 	ctags of SVR4 Unix.
 
 ``-x``
+	Same as ``--output-format=xref``.
 	Print a tabular, human-readable cross reference (xref) file to standard
 	output instead of generating a tag file. The information contained in
 	the output includes: the tag name; the kind of tag; the line number,
@@ -1389,6 +1390,17 @@ are not listed here. They are experimental or for debugging purpose.
 	Add an optlib *directory* to or reset **optlib** path list.
 	By default, the optlib path list is empty.
 
+``--output-format=u-ctags|e-ctags|etags|xref|json``
+	Specify the output format. The default is "u-ctags".
+	See tags(5) for "u-ctags" and "e-ctags".
+	See ``-e`` for "etags", and ``-x`` for "xref".
+	"json" is experimental format, and available only if
+	the ctags executable is built with libjansson.
+	This option must appear before the first file name.
+
+.. TODO: convert output-json.rst to ctags-json-output.1.rst (ctags-json-output(1)).
+   and add a link to it here.
+
 ``--print-language``
 	Just prints the language parsers for specified source files, and then exits.