Fix grammar error in py_features_meaning.markdown (#8378)

`template<typename _Tp> inline const _Tp* Mat_<_Tp>::operator [](int y) const` does not support 3d matrix since it checks rows.

This operator[] shall check size.p[0] instead.

Make it more clear for the type of input argument.

Current version of search for MLKROOT environment variable is wrong and should be corrected to (DEFINED ENV{MKLROOT})

Synchronizing against non-final fields is advised against.

* Use the YV12 format in the Android emulator to avoid image issues

* Removed trailing spaces

* Added exception in else case

* Removed tab