Document `occ app:..` --flags and usage examples

Documentation doesn't cover functionality of occ commands (such as enabling an app for specific groups), which could confuse admin user while attempting to set up the instance. Flag --help is providing documented flag, but without usage examples

root@bfd4c603dbb2:/var/www/html# occ app:enable --help
Description:
  enable an app

Usage:
  app:enable [options] [--] <app-id>...

Arguments:
  app-id                enable the specified app

Options:
  -g, --groups=GROUPS   enable the app only for a list of groups (multiple values allowed)
  -f, --force           enable the app regardless of the Nextcloud version requirement
  -h, --help            Display help for the given command. When no command is given display help for the list command
  -q, --quiet           Do not output any message
  -V, --version         Display this application version
      --ansi|--no-ansi  Force (or disable --no-ansi) ANSI output
  -n, --no-interaction  Do not ask any interactive question
      --no-warnings     Skip global warnings, show command output only
  -v|vv|vvv, --verbose  Increase the verbosity of messages: 1 for normal output, 2 for more verbose output and 3 for debug