docs: clarify description key is optional for composite actions

[Xiyu-z]

Mark description field as optional for composite actions and add recommendation note.

Why:

The documentation previously stated the description field was required for composite actions, but composite actions can run without this field. This update clarifies that it is optional while still recommending users to provide it.

Closes #43910

What's being changed:

Changed "Required" to "Optional" for the description metadata field under composite actions, and added a note that it is highly recommended to include a description.

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content
• All CI checks are passing and the changes look good in the review environment.

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Source	Review	Production	What Changed
actions/reference/workflows-and-actions/metadata-syntax.md	fpt ghec ghes@ 3.22 3.21 3.20 3.19 3.18	fpt ghec ghes@ 3.22 3.21 3.20 3.19 3.18

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[samyelzoghby01-lang]

content/actions/reference/workflows-and-actions/metadata-syntax.md