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

[Antreesy]

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

[Duo123418]

Hello! Can I solve this issue?

[Antreesy]

Hello! Can I solve this issue?

Hi! Sure, make a PR)