rephrase and correct the descriptions for clSetKernelExecInfo

[bashbaug]

fixes #1152 fixes #1244

Rephrases and corrects the descriptions for multiple parts of clSetKernelExecInfo.

Removes the cumbersome "note" below the API description and moves the content into the API description instead. I don't think I lost any important content, but this would be a good area for a careful review.

Explicitly states that an empty set of SVM pointers for CL_KERNEL_EXEC_INFO_SVM_PTRS is valid.

[bashbaug]

Depending how we resolve #1244 we may need to make additional clarifications.

[bashbaug]

Converting to draft to work out system SVM updates.

[bashbaug]

This is ready for review again.

[bashbaug]

Merging (offline) as discussed in the October 22nd teleconference.