add help text to spectacular management command arguments #1175

Each spectacular flag (--format, --urlconf, --generator-class, --file,
--fail-on-warn, --validate, --api-version, --lang, --color,
--custom-settings) now carries a help= description so that
`./manage.py spectacular --help` is self-documenting.

Includes a regression test asserting every spectacular-defined flag
carries non-empty help text.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>