docs/bpf: Add description for CO-RE relocations

Add a section on CO-RE relocations to llvm_relo.rst. Describe relevant .BTF.ext
structure, `enum bpf_core_relo_kind` and `struct bpf_core_relo` in some detail.

Description is based on doc-strings from:

  - include/uapi/linux/bpf.h:struct bpf_core_relo
  - tools/lib/bpf/relo_core.c:__bpf_core_types_match()
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/bpf/20230826222912.2560865-2-eddyz87@gmail.com

Documentation/bpf/btf.rst

@@ -726,8 +726,8 @@ same as the one describe in :ref:`BTF_Type_String`.
 4.2 .BTF.ext section
 --------------------
 
-The .BTF.ext section encodes func_info and line_info which needs loader
-manipulation before loading into the kernel.
+The .BTF.ext section encodes func_info, line_info and CO-RE relocations
+which needs loader manipulation before loading into the kernel.
 
 The specification for .BTF.ext section is defined at ``tools/lib/bpf/btf.h``
 and ``tools/lib/bpf/btf.c``.
@@ -745,15 +745,20 @@ The current header of .BTF.ext section::
         __u32   func_info_len;
         __u32   line_info_off;
         __u32   line_info_len;
+
+        /* optional part of .BTF.ext header */
+        __u32   core_relo_off;
+        __u32   core_relo_len;
     };
 
 It is very similar to .BTF section. Instead of type/string section, it
-contains func_info and line_info section. See :ref:`BPF_Prog_Load` for details
-about func_info and line_info record format.
+contains func_info, line_info and core_relo sub-sections.
+See :ref:`BPF_Prog_Load` for details about func_info and line_info
+record format.
 
 The func_info is organized as below.::
 
-     func_info_rec_size
+     func_info_rec_size              /* __u32 value */
      btf_ext_info_sec for section #1 /* func_info for section #1 */
      btf_ext_info_sec for section #2 /* func_info for section #2 */
      ...
@@ -773,7 +778,7 @@ Here, num_info must be greater than 0.
 
 The line_info is organized as below.::
 
-     line_info_rec_size
+     line_info_rec_size              /* __u32 value */
      btf_ext_info_sec for section #1 /* line_info for section #1 */
      btf_ext_info_sec for section #2 /* line_info for section #2 */
      ...
@@ -787,6 +792,20 @@ kernel API, the ``insn_off`` is the instruction offset in the unit of ``struct
 bpf_insn``. For ELF API, the ``insn_off`` is the byte offset from the
 beginning of section (``btf_ext_info_sec->sec_name_off``).
 
+The core_relo is organized as below.::
+
+     core_relo_rec_size              /* __u32 value */
+     btf_ext_info_sec for section #1 /* core_relo for section #1 */
+     btf_ext_info_sec for section #2 /* core_relo for section #2 */
+
+``core_relo_rec_size`` specifies the size of ``bpf_core_relo``
+structure when .BTF.ext is generated. All ``bpf_core_relo`` structures
+within a single ``btf_ext_info_sec`` describe relocations applied to
+section named by ``btf_ext_info_sec->sec_name_off``.
+
+See :ref:`Documentation/bpf/llvm_reloc <btf-co-re-relocations>`
+for more information on CO-RE relocations.
+
 4.2 .BTF_ids section
 --------------------