Add `description` kwarg to command.Parameter and show in help command

[Soheab]

Summary

This PR adds a way to set a description for an ext.commands/hybrid command argument. and reformats the DefaultHelpCommand to show that description, MinimalHelpCommand is left unchanged.

Also added arguments_heading and default_argument_description keyword arguments to DefaultHelpCommand.

Checklist

• [x] If code changes were made then they have been tested.
  • [x] I have updated the documentation to reflect the changes.
• [ ] This PR fixes an issue.
• [x] This PR adds something new (e.g. new method or parameters).
• [ ] This PR is a breaking change (e.g. methods or parameters removed/renamed)
• [ ] This PR is not a code change (e.g. documentation, README, ...)