GH-92892: Add section about variadic functions to ctypes documentation (GH-99529)

On some platforms, and in particular macOS/arm64, the calling
convention for variadic arguments is different from the regular
calling convention. Add a section to the documentation to document
this.
(cherry picked from commit bc3a11d21ddef28047b18c0f6a5068fa9fb16da2)

Co-authored-by: Ronald Oussoren <ronaldoussoren@mac.com>