Describe the doc issue

Doc link for haveImageReader(): https://docs.opencv.org/4.11.0/d4/da8/group__imgcodecs.html#ga65d3569d8845d1210e1aeab8c199031c

The note states It does not check for the actual existence of the file but rather the ability to read the specified file type, which is incorrect. haveImageReader() calls findDecoder(), which attempts to actually open the file. If the file is not present, a warning message is printed and haveImageReader() returns False, even if the format truly is supported. Testing in Python on my machine (where .png definitely is supported):

>>> havePNG = cv2.haveImageReader(".png")
[ WARN:0@537.189] global loadsave.cpp:268 cv::findDecoder imread_('.png'): can't open/read file: check file path/integrity
>>> print(havePNG)
False

Fix suggestion

Can change the note to say something like:

"The file must be present. To check for an encoder/decoder without needing a file to exist, it's best to use haveImageWriter(). The encoders and decoders come in pairs, so the existence of an image writer implies the existence of the corresponding image reader."

And maybe do a little more fact checking with the output of ChatGPT: #25888 😉