Synchronize documentation for `add_filter()`, `tests_add_filter()`, `_wp_filter_build_unique_id()`, `_test_filter_build_unique_id()`.
Add a note that `$tag` and `$priority` are no longer used in `_wp_filter_build_unique_id()` since [46220], and the function always returns a string now.
Props donmhico, remcotolsma, SergeyBiryukov.
Fixes#47407. See #48303.
git-svn-id: https://develop.svn.wordpress.org/trunk@46801 602fd350-edb4-49c9-b593-d223f7449a82
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