cl_khr_external_memory_dma_buf: Clarify usage (KhronosGroup#1335)

* cl_khr_external_memory_dma_buf: Clarify usage

* add asciidoctor attribute formatting.

---------

Co-authored-by: Ben Ashbaugh <ben.ashbaugh@intel.com>

Author: joshqti

api/opencl_runtime_layer.asciidoc

@@ -6142,7 +6142,12 @@ handles, and adds as a property that may be specified when creating a buffer
 or an image memory object from an external handle:
 
   * {CL_EXTERNAL_MEMORY_HANDLE_DMA_BUF_KHR_anchor} is a file descriptor for a Linux
-    dma_buf.
+    dma_buf. Once a memory object created from a dma_buf has been acquired with 
+    {clEnqueueAcquireExternalMemObjectsKHR}, all access, including host access should 
+    be done through OpenCL, until a call to {clEnqueueReleaseExternalMemObjectsKHR} has completed.
+    Any operations external to OpenCL on the dma_buf while it is acquired 
+    may result in undefined behavior.
+    
     It owns a reference to the underlying memory resource represented by its
     memory object.
 endif::cl_khr_external_memory_dma_buf[]