fixes #19396; Nimdoc hide nonexported fields (#21305)

* suppresses non-exported fields of types and adds command-line option to re-enable this if desired

* corrected the doctest that produced a CI error

* an embarrassingly bad error in reasoning

* modified a nimdoc test to reflect updated behavior

* needed another change to bring utils.html doctest in sync with update

* add info

* fix nimdoc

* lint

* render postfix

* fixes a problem

* fixes nimdoc

* fix nimdoc

---------

Co-authored-by: johnperry-math <john.perry@usm.edu>
Co-authored-by: johnperry-math <devotus@yahoo.com>

compiler/commands.nim

@@ -1008,6 +1008,9 @@ proc processSwitch*(switch, arg: string, pass: TCmdLinePass, info: TLineInfo;
     expectNoArg(conf, switch, arg, pass, info)
     conf.exc = low(ExceptionSystem)
     defineSymbol(conf.symbols, "noCppExceptions")
+  of "shownonexports":
+    expectNoArg(conf, switch, arg, pass, info)
+    showNonExportedFields(conf)
   of "exceptions":
     case arg.normalize
     of "cpp": conf.exc = excCpp