Correcting return type from videoDevice::findType to be int thus allowing detection of failed device init request

  * loosen the threshold only under aarch64
  * fix #6610

fix for #6598

at least it is possible to read/ write calibration files. Fixes #4282.

Also add CPP method for writing comments.

While generating Java JNI wrappers package names with an underscore (`_`) character where not properly escaped according to https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/design.html#resolving_native_method_names (see also: https://github.com/Itseez/opencv_contrib/issues/652).

This fix replaces all the occurrences of `_` with `_1` resulting in proper JNI method names.

update documentation of calcBackProject() calcHist() compareHist()