Before, we had the focus on attributes and how they were impacted by
various features. Now we separate out language items and put both magic
and raw attributes under the type of attribute (command, arg, etc)
For the derive API, you can only call `next_display_order` when dealing
with a flatten. Until we offer app attributes on arguments, the user can workaround with
this no-op flattens.
This is a part of #1807
This clarifies the intent and prepares for other functions doing the
same, like `next_display_order`. This will then open us to name
`subcommand_help_heading` and `display_order` similar.
The deprecation is waiting on 3.1.
This is part of #1807 and #1553.
`#[clap(about)]` only overrides `about`. If the doc comment also sets
`long_about`, it won't be overridden. This change is to help raise
visibility of reseting `long_about` in these cases.
While I'm unsure how much type specialization we should do, we
intentionally have the `arg_enum` attribute for doing special behavior
based on it, so let's take advantage of it.
Fixes#3185
