cleanup documentation and some extension handling

[bashbaug]

This is a slightly reduced version of #148: it removes the extension defines for the cl_ext_device_fission and cl_khr_image2d_from_buffer extensions because they do not require significant code restructuring, but leaves the extension defines for cl_khr_il_program and cl_khr_subgroups.

After we get in these changes we can look into improving extension handling in general with a subsequent PR.

Other related improvements:

• Corrects the documentation for CL_HPP_TARGET_OPENCL_VERSION and CL_HPP_MINIMUM_OPENCL_VERSION.
• The query for CL_DEVICE_PARENT_DEVICE_EXT should return a cl::Device not a cl_device_id, similar to the query for CL_DEVICE_PARENT_DEVICE.
• When querying for extension funciton pointers, first call clGetExtensionFunctionAddressForPlatform if compiling for OpenCL 1.2 or newer, then call clGetExtensionFunctionAddress if compiling for pre-OpenCL 1.2.

[bashbaug]

Merging - this has two separate approvals now.