Refine OpenCV documentation

* Warn if features are only supported with patched OpenCV.
* Clarify what is meant by "in-place"
* In-place *is* supported for resize, transpose & Sobel because OpenCV
  will create a new image before calling the HAL.
* Note optimal sigma values for Gaussian blur.
* Note the need to specify accuracy hint for Gaussian blur.
* Add missing constraints on element size for transpose.
* Fix references to non-existent src & dst parameters.
* Use Mat's type instead of both depth and channels.
* ^ means bitwise xor in C, so don't use it to mean power.