doc: update libxdp usage in af_xdp guide

doc: update libxdp usage in af_xdp guide

When libxdp is used, the LIBXDP_OBJECT_PATH environment variable must be
set to the location of where libxdp placed its bpf object files. This is
usually in /usr/local/lib/bpf or /usr/local/lib64/bpf. Failure to do so
will result in the PMD not initialising correctly as the bpf program is
not found. Document this requirement.

Also, mention that the following logs which are generated on application
launch can be ignored:

  libbpf: elf: skipping unrecognized data section(7) .xdp_run_config
  libbpf: elf: skipping unrecognized data section(8) xdp_metadata

Signed-off-by: Ciara Loftus <ciara.loftus@intel.com>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>