[3.12] gh-87969: Align docs and docstrings with implementation for ctypes' [w]string_at() (GH-25384) (GH-118046)

gh-87969: Align docs and docstrings with implementation for ctypes' [w]string_at() (GH-25384)

The implementation uses 'ptr' for the name of the first parameter of
ctypes.string_at() and ctypes.wstring_at(). Align docs and docstrings
with the naming used in the implementation.

(cherry picked from commit 81a926bd20a8c66646e51b66ef1cfb309b73ebe7)

Co-authored-by: Shreyan Avigyan <shreyan.avigyan@gmail.com>
Co-authored-by: Irit Katriel <1055913+iritkatriel@users.noreply.github.com>
Co-authored-by: Erlend E. Aasland <erlend.aasland@protonmail.com>