imgproc: update cornerSubPix documentation

Fix for SSE2 intrinsics problem in the part of saturation arithmetic processing during 32s->16u packed conversion -
for some big negative values less than -INT_MAX+32767 the sign of the numbers is lost due to overflow that leads to
incorrect saturation to MAX value, instead of zero.
The issue is not reproduced with CV_ENABLED_INTRINSICS=OFF

- tests for both SIMD128 / SIMD256
- different dispatched + baseline(SIMD128) intrinsics

Found via `codespell -q 3 -I ../opencv-whitelist.txt --skip="./3rdparty"`