cl_khr_command_buffer access to read-only memory objects

clCommandFillImageKHR and similar should restrict target images to be write or read-write. clEnqueueFillImage permits fills of read-only images, which may not be implemented in a performant way in the context of command buffers.

Locate the specification text for clCommandFillImageKHR and related command-buffer fill functions, then compare their image-access requirements with clEnqueueFillImage. Update the documented requirements so read-only target images are handled consistently, and verify the affected extension text and related references.

Written by the indexing model from the issue text.