Docs: Correct `@param` type for the function parameter in `tests_add_filter()`...

Docs: Correct `@param` type for the function parameter in `tests_add_filter()` and `_test_filter_build_unique_id()`.

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.
Built from https://develop.svn.wordpress.org/trunk@46801


git-svn-id: https://core.svn.wordpress.org/trunk@46601 1a063a9b-81f0-0310-95a4-ce76da25c4cd