Sharing: Add filter docblock for is_jetpack_site

[aduth]

As mentioned by @jeherve at #1946 (comment), new filter usage should include a DocBlock or DocBlock reference. This pull request adds a DocBlock definition for the is_jetpack_site filter usage in sharing-service.php. The filter is used elsewhere already, and other updates will need to be made.

General questions related to filter documenting:

• How do we reconcile WordPress.com file syncing for referenced DocBlocks?
• Should we prefer to document add_filter or apply_filter usage?
• Is there a guideline on which filter to prefer the DocBlock be fully described (not referenced)?

[jeherve]

How do we reconcile WordPress.com file syncing for referenced DocBlocks?

That's something we can handle with the merge script, I think.

Should we prefer to document add_filter or apply_filter usage?

apply_filters.
https://make.wordpress.org/core/handbook/inline-documentation-standards/php-documentation-standards/#4-hooks-actions-and-filters

Is there a guideline on which filter to prefer the DocBlock be fully described (not referenced)?

Preferably the place where it was first introduced, then follow core's way of referencing duplicate filters:
https://make.wordpress.org/core/handbook/inline-documentation-standards/php-documentation-standards/#4-1-duplicate-hooks