Improve help for app config link flags

[gonzaloriestra]

WHY are these changes introduced?

The shared --config description doesn't explain how app config link uses the local configuration. When combined with --client-id, the generic flag conflict error also doesn't explain how to choose the destination file.

WHAT is this pull request doing?

Clarify the command's --config, --client-id, and --file-name descriptions. Show a custom error recommending --file-name when both --config and --client-id are provided, before attempting to link the app. Cover command-line flags and environment variables, and refresh the generated command manifest, README, and docs.

How to manually test your changes?

pnpm shopify app config link --help
pnpm shopify app config link --config staging --client-id example-client-id

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes
• I've considered analytics changes to measure impact
• The change is user-facing — I've identified the correct bump type (patch for bug fixes · minor for new features · major for breaking changes) and added a changeset