Document help page generation for maintainers

[Rowlando13]

After reading the code, it is not clear to me how exactly help pages are generated. Before any changes can be considered, we should have a shared understanding of the exact path for generating help pages.

At some point, I will play around in a debugger and work this out. If anyone knows, feel free to chime in.

[gdesai23]

I traced the current --help and no_args_is_help flows locally and opened a PR documenting how help is assembled and rendered today.
PR: #3326
@Rowlando13