Suppress private visibility arguments in generate-manual output

[monoxygen]

Arguments that specify private visibility, such as:

@Flag(help: ArgumentHelp(visibility: .private))
var hiddenFromHelp: Bool = false

show up in the man page generated by the generate-manual command:

.It Fl -hidden-from-help

If private visibility indicates that help should never be shown for that argument, it seems reasonable and desirable that it also not be visible in man pages.

ArgumentParser version: 1.5.0
Swift version: swift-driver version: 1.110 Apple Swift version 6.0 (swiftlang-6.0.0.4.52 clang-1600.0.21.1.3)

Checklist

• If possible, I've reproduced the issue using the main branch of this package
• I've searched for existing GitHub issues

Steps to Reproduce

1. Define an argument with private visibility, such as the hiddenFromHelp argument above
2. Build the client executable at path <executable>
3. Run swift run generate-manual <executable> --output-directory <outputdir>

Expected behavior

hiddenFromHelp should not be included in the man page generated at <outputdir>

Actual behavior

hiddenFromHelp is included in the man page generated at <outputdir>

[heckj]

Saw the bug listing while I was digging around the repo related to dumping the help output. The arguments are annotated with visibility information up front, but it wasn't evaluated when generating the manual output, so my PR (#667) checks that additional property before generating relevant content. The existing Math example in the test output included "hidden" content, so I've updated the tests to match - the arguments marked as hidden shouldn't be in the resulting output.

I think I've matched style, etc - but more than happy to tweak or change if something is amiss or isn't following a preferred pattern for this additional check.