`cl_intel_fpga_host_pipe` extension defines enums, but is missing everything else

These enums are defined, but not used in any `` blocks of `cl.xml`:

- `CL_KERNEL_ARG_HOST_ACCESSIBLE_PIPE_INTEL`
- `CL_DEVICE_MAX_HOST_READ_PIPES_INTEL`
- `CL_DEVICE_MAX_HOST_WRITE_PIPES_INTEL`
- `CL_PIPE_FULL`
- `CL_PIPE_EMPTY`

They seem to be related to this extension:
https://www.iwocl.org/wp-content/uploads/iwocl-2018-intel-michael-kinsner-host-pipes.pdf

Is there something stopping this extension from being added to this repo?
I can make PR to add the `` blocks, but there is also no .txt or .adoc file to describe it.

Start by inspecting the enum definitions and their surrounding entries in cl.xml, then compare the proposed extension with the linked Intel host-pipes PDF. Determine whether the extension should be documented here and whether a .txt or .adoc description is required; done means the accepted extension has the appropriate require blocks and description source.

Written by the indexing model from the issue text.