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

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