Clarify 'description' key is optional for composite actions

### Code of Conduct

- [x] I have read and agree to the GitHub Docs project's [Code of Conduct](https://github.com/github/docs/blob/main/.github/CODE_OF_CONDUCT.md)

### What article on docs.github.com is affected?

https://docs.github.com/en/actions/reference/workflows-and-actions/metadata-syntax

### What part(s) of the article would you like to see updated?

For composite actions, the `description` key is not strictly required in the `action.yml` for the composite action to function correctly (see Additional Information for proof).

Instead of **Required** this should either have an addendum like in the pull request (that is, that `description` is not required for composite actions, or change **Required** to optional. In either case, it is probably worth it to state that a `description` key is highly recommended.

### Additional information

I just ran a workflow run that calls a composite action whose `action.yml` file did not include a description key at the time this workflow ran. The workflow ran as expected (all the composite action does is produce an output that gets passed to the calling workflow).

See https://github.com/mcummings128/gh-actions-outputs-experiments-1/actions/runs/24743176941 for details.
This commit SHA shows the description key was removed from the composite action during the time the workflow ran.
https://github.com/mcummings128/gh-actions-outputs-experiments-1/commit/6037d82f9910c7d186845393ab2b4a53617b2bde

Lastly, please see the pull request this is linked to: https://github.com/github/docs/pull/43911

從 docs.github.com/en/actions/reference/workflows-and-actions/metadata-syntax 的中繼資料語法文章開始，並查看連結的 pull request，核對提議的措辭。更新 composite-action 描述要求，以反映該描述是選用的，同時說明強烈建議提供該描述，然後驗證文章是否能正確呈現。

由索引模型根據 Issue 內容生成。