bpo-39153: Clarify C API *SetItem refcounting semantics (GH-18220)

Some of the *SetItem methods in the C API steal a reference to the
given value. This annotates the better behaved ones to assure the
reader that these are not the ones with the inconsistent behaviour.

* 📜🤖 Added by blurb_it.

* make docs consistent with signature

Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>