Add docs for `--download-migration-logs` option to `generate-script` command

[synthead]

Introduced with https://github.com/github/gh-gei/pull/382! Plese let the PR :point_left: for these options to be reviewed and merged before starting this issue, as the options may change :sparkles:

Add documentation for the --download-migration-logs option to generate-script command.

Truncated help page from gei generate-script --help with this new option:

generate-script
  Generates a migration script. This provides you the ability to review the steps that this tool will take, and 
  optionally modify the script if desired before running it.

Usage:
  gei [options] generate-script

Options:
  ...
  --download-migration-logs                           Downloads the migration log for for each repostiory 
                                                      migration.
  ...