doc/utils: apply misc fixes to OpenCL docs

doc/utils.texi

@@ -565,20 +565,27 @@ indication of the corresponding powers of 10 and of 2.
 @c man begin OPENCL OPTIONS
 
 When FFmpeg is configured with @code{--enable-opencl}, it is possible
-to set the options to set in the global OpenCL context. The list of
-supported options follows:
+to set the options for the global OpenCL context.
+
+The list of supported options follows:
 
 @table @option
 @item build_options
-Set build options which used to compiled kernels, see reference "OpenCL Specification Version: 1.2 chapter 5.6.4"
+Set build options used to compile the registered kernels.
+
+See reference "OpenCL Specification Version: 1.2 chapter 5.6.4".
 
 @item platform_idx
-Select platform to run OpenCL code, the platform_idx is the index of platform
-in the device list which can be obtained with av_opencl_get_device_list().
+Select the index of the platform to run OpenCL code.
+
+The specified index must be one of the indexes in the device list
+which can be obtained with av_opencl_get_device_list().
 
 @item device_idx
-Select device to run OpenCL code, the device_idx is the index of device in
-the device list which can be obtained with av_opencl_get_device_list().
+Select the index of the device used to run OpenCL code.
+
+The specifed index must be one of the indexes in the device list which
+can be obtained with av_opencl_get_device_list().
 
 @end table