Fix typos in the documentation for AutoBuffer. (#8197)

* Allocate 1000 floats to match the documentation

Fix the documentation of `AutoBuffer`. By default, the following code
```.cpp
cv::AutoBuffer<float> m;
```` 
allocates only 264 floats. But the comment in the demonstration code says it allocates 1000 floats, which is 
not correct.

* fix typo in the comment.

cv2.findContours() no longer modifies source image (#8150)

* cv2.findContours() no longer modifies source image

Since OpenCV 3.2, cv2.findContours() no longer modifies the source image but returns a modified image as the first of three return parameters.  ??? Do I have that correct ???

Also fixed up the code blocks to be valid markdown.

* ```python --> @code{.py}

Enables syntax highlighting in docs.

taken from https://github.com/google/googletest/pull/856

Solve issue 8136 opencv doc of threshold

Add a link to MatExpr in Detailed Description of Mat

The backslash causes build failure when cross-compiling with mingw-w64

fix for opencv/opencv#8105, compilation issue with mingw32 (in
google/googletest#721 a similar issue was solved and the reason was
described as MinGW defines _CRITICAL_SECTION and _RTL_CRITICAL_SECTION
as two separate (equivalent) structs, instead of using typedef)

CoInitialize/CoUninitialize in VFW classes (#8112)