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

Updating btf.rst doc with info about BTF_SET_START/END macros.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200825192124.710397-12-jolsa@kernel.org

Documentation/bpf/btf.rst

@@ -724,6 +724,31 @@ want to define unused entry in BTF_ID_LIST, like::
       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.