Issue Description:

The help information emitted for the docc index subcommand instructs the user to provide a .docc catalog as input. The command actually expects a Data folder of render JSON files from a compiled documentation archive. We should update the help information to reflect this.

Currently this is what docc emits for ‘docc index --help’:

OVERVIEW: Create an index for the documentation from compiled data.

USAGE: docc index <source-bundle-path> -bundle-identifier <bundle-identifier> [-verbose]

ARGUMENTS:
<source-bundle-path> Path to a documentation bundle directory.
The '.docc' bundle docc will build.