Respect flatten_help in man pages

[epage]

#5206 added support for subcommands like git stash that summarize all of their subcommands. Unlike git, we show short help on git stash --help for these subcommands and long help on git stash pop --help.

The question is how these should be handled in man pages.

Example idea:

• Flattened subcommands have their long help embedded into their parent command's man page and generate skips generating files for these (see feat(mangen): generate filename and files #5331)