Contextual usage information needs improvement

[rzumer]

Usage information is displayed in a context-sensitive manner, but it is somewhat inconsistent:

• Calling rav1e -h will provide a full list of arguments with information on each one
• Omitting required arguments (input and output files) will display an example that excludes some optional arguments (RECONSTRUCTION)
• Providing an invalid value for an optional argument will display an example that includes only required arguments and the argument that was not parsed correctly, with insufficient information (no description).

It is always possible to fall back to the -h or --help command, but contextual information could be improved.

[shreevari]

I want to take this up.
I checked out the code, but could not figure out why clap is not displaying some optional arguments(like RECONSTRUCTION), but I can add contextual information as description when optional arguments are not parsed correctly.