Unlike `PySequence_GetItem`, `PyTuple_GetItem` does not support negative indices ("indexing from the end").  That is fine, but warrants a notice in the docs (as that behavior is certainly not obvious).
The same wording as for `PyList_GetItem` (changing "list" to "tuple") should be good enough (`PyList_GetItem` currently states: "The position must be non-negative; indexing from the end of the list is not supported. If index is out of bounds (<0 or >=len(list)), return NULL and set an IndexError exception.")