bpf: Update .BTF_ids section in btf.rst with sets info

      BTF_ID_UNUSED

      BTF_ID(struct, task_struct)

The ``BTF_SET_START/END`` macros pair defines sorted list of BTF ID values

and their count, with following syntax::

  BTF_SET_START(set)

  BTF_ID(type1, name1)

  BTF_ID(type2, name2)

  BTF_SET_END(set)

resulting in following layout in .BTF_ids section::

  __BTF_ID__set__set:

  .zero 4

  __BTF_ID__type1__name1__3:

  .zero 4

  __BTF_ID__type2__name2__4:

  .zero 4

The ``struct btf_id_set set;`` variable is defined to access the list.

The ``typeX`` name can be one of following::

   struct, union, typedef, func

and is used as a filter when resolving the BTF ID value.

All the BTF ID lists and sets are compiled in the .BTF_ids section and

resolved during the linking phase of kernel build by ``resolve_btfids`` tool.