docs: explain how to set attachment image alt text

[DeoJin]

Summary

• document how to set attachment image �lt text in the rix-attachment-add handler
• clarify that attachment captions do not automatically become the image �lt attribute
• point readers to setAttributes for cases where they want both

Closes #1230.

Validation

• git diff --check -- README.md
• attempted targeted accessibility coverage with
  ode .\node_modules\@web\test-runner\dist\bin.js src/test/system/accessibility_test.js --playwright --browsers chromium after
  pm install, but the local environment hit dependency resolution errors under Node 24 before the suite could start