Photon: update filter docblock to match possible types

[jeherve]

Proposed changes:

If you browse through that file, you will see that we can pass a string, an array, or even null to jetpack_photon_skip_image. Let's update the docblock to reflect that.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

• N/A

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

• No

Testing instructions:

Not much to test here. I would only recommend to browse for usage of jetpack_photon_skip_image and see that it matches the updated docblock.

[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WordPress.com Simple site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin, and enable the update/photon-filter-doc branch.

• To test on Simple, run the following command on your sandbox:

  bin/jetpack-downloader test jetpack update/photon-filter-doc
  

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[github-actions]

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