possible clarifications and consistency fixes for row pitch and slice pitch

The fix for issue #624 added an error condition for the slice pitch for clEnqueueReadImage and clEnqueueWriteImage but a few related parts of the spec could still use attention:

* Should there be a requirement that the `row_pitch` or `input_row_pitch` must be zero?
* The currently descriptions are slightly contradictory and say that e.g. the value "must be greater than or equal to the element size in bytes × width" but then that if "row_pitch is set to 0, the appropriate row pitch is calculated". It would be better to say that the value must be zero **or** greater than the proper value in these cases.

I initially thought similar clarifications could be needed for the row pitch and slice pitch arguments to `clEnqueueReadBufferRect`, `clEnqueueWriteBufferRect`, and `clEnqueueCopyBufferRect` would be needed, but after looking more closely I _think_ they're OK.

Start by locating the OpenCL specification descriptions for clEnqueueReadImage and clEnqueueWriteImage, then review the fix for issue #624. Determine whether row_pitch and input_row_pitch may be zero, and make the zero-or-minimum wording consistent with the slice-pitch text. Confirm whether the rect buffer APIs require any changes.

Written by the indexing model from the issue text.