bpftool/docs: add description of btf dump C option

Document optional **c** option for btf dump subcommand.

Cc: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/bpf/bpftool/Documentation/bpftool-btf.rst

@@ -19,10 +19,11 @@ SYNOPSIS
 BTF COMMANDS
 =============
 
-|	**bpftool** **btf dump** *BTF_SRC*
+|	**bpftool** **btf dump** *BTF_SRC* [**format** *FORMAT*]
 |	**bpftool** **btf help**
 |
 |	*BTF_SRC* := { **id** *BTF_ID* | **prog** *PROG* | **map** *MAP* [{**key** | **value** | **kv** | **all**}] | **file** *FILE* }
+|	*FORMAT* := { **raw** | **c** }
 |	*MAP* := { **id** *MAP_ID* | **pinned** *FILE* }
 |	*PROG* := { **id** *PROG_ID* | **pinned** *FILE* | **tag** *PROG_TAG* }
 
@@ -49,6 +50,10 @@ DESCRIPTION
 		  .BTF section with well-defined BTF binary format data,
 		  typically produced by clang or pahole.
 
+		  **format** option can be used to override default (raw)
+		  output format. Raw (**raw**) or C-syntax (**c**) output
+		  formats are supported.
+
 	**bpftool btf help**
 		  Print short help message.