Improve PHPDoc clarity for jetpack_post_sharing_get_value

[subhampatra2605]

Improves the PHPDoc description for the jetpack_post_sharing_get_value callback to clarify how the effective sharing state is determined, without changing functionality.

[github-actions]

Thank you for your PR!

When contributing to Jetpack, we have a few suggestions that can help us test and review your patch:

• 🔴 Include a description of your PR changes.
  
• 🔴 Add testing instructions.
  
• 🔴 Specify whether this PR includes any changes to data or privacy.
  
• 🔴 Add changelog entries to affected projects
  

This comment will be updated as you work on your PR and make changes. If you think that some of those checks are not needed for your PR, please explain why you think so. Thanks for cooperation 🤖

---

🔴 Action required: Please edit your PR description and explain what functional changes your PR includes, and why those changes are needed.

---

🔴 Action required: Please include detailed testing steps, explaining how to test your change, like so:

## Testing instructions:

* Go to '..'
*

---

🔴 Action required: We would recommend that you add a section to the PR description to specify whether this PR includes any changes to data or privacy, like so:

## Does this pull request change what data or activity we track or use?

My PR adds *x* and *y*.

---

🔴 Action required: Please add missing changelog entries for the following projects: projects/plugins/jetpack

Use the Jetpack CLI tool to generate changelog entries by running the following command: jetpack changelog add.
Guidelines: /docs/writing-a-good-changelog-entry.md

---