Provide documentation to indicate changes to wp_privacy_exports_url require moving files to avoid breaking existing exports

[garrett-eclipse]

This is a follow-up of #44038 which was committed in 5.5 as [48127] and [48330]. That ticket updated the handling of privacy export paths making them relative and using the wp_privacy_exports_url(). This function provides the wp_privacy_exports_url which allows admins to customize the location of their exports. As of the commits changing this location via the filter will break any existing exports unless they're migrated to the new directory location. As such this ticket strives to educate this via the filters through a new since phpdoc (patch coming).

Note: This is a potential candidate for 5.5.2 but for now I'll milestone for the next major.

[garrett-eclipse]

Added @since docs to the exports dir/url filters to indicate changes should be reflected on the server to avoid breaking existing requests.

[SergeyBiryukov]

In 49042:

Docs: Add a @since note to wp_privacy_exports_dir and wp_privacy_exports_url filters about exports using relative paths since WordPress 5.5.

When changing exports location via these filters, make sure to migrate the files to the new directory, to avoid breaking any existing exports.

Follow-up to [48127], [48330].

Props garrett-eclipse.
Fixes #51361.