Update note on action description requirement

[mcummings128]

Clarified that the description is not required for composite actions.

As a proof of concept, I deleted my 'description' key from my composite action and called it, with everything still working.

Why:

Closes: #43910

What's being changed (if available, include any code snippets, screenshots, or gifs):

Per the issue linked above:

"For composite actions, the description key is not strictly required for the composite action to function correctly (see Additional Information [in the issue] 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."

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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