bpo-29636: json.tool: Add document for indentation options. (GH-17482)

And updated test to use subprocess.run
diff --git a/Doc/library/json.rst b/Doc/library/json.rst
index 573ec1c..cfe68c9 100644
--- a/Doc/library/json.rst
+++ b/Doc/library/json.rst
@@ -744,6 +744,12 @@
 
    .. versionadded:: 3.8
 
+.. cmdoption:: --indent, --tab, --no-indent, --compact
+
+   Mutually exclusive options for whitespace control
+
+   .. versionadded:: 3.9
+
 .. cmdoption:: -h, --help
 
    Show the help message.