bpftool: Update doc to explain struct_ops register subcommand.

The "struct_ops register" subcommand now allows for an optional *LINK_DIR*
to be included. This specifies the directory path where bpftool will pin
struct_ops links with the same name as their corresponding map names.

Signed-off-by: Kui-Feng Lee <kuifeng@meta.com>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/r/20230420002822.345222-2-kuifeng@meta.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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

@@ -26,7 +26,7 @@ STRUCT_OPS COMMANDS
 
 |	**bpftool** **struct_ops { show | list }** [*STRUCT_OPS_MAP*]
 |	**bpftool** **struct_ops dump** [*STRUCT_OPS_MAP*]
-|	**bpftool** **struct_ops register** *OBJ*
+|	**bpftool** **struct_ops register** *OBJ* [*LINK_DIR*]
 |	**bpftool** **struct_ops unregister** *STRUCT_OPS_MAP*
 |	**bpftool** **struct_ops help**
 |
@@ -51,10 +51,14 @@ DESCRIPTION
 		  for the given struct_ops.  Otherwise, it dumps all struct_ops
 		  currently existing in the system.
 
-	**bpftool struct_ops register** *OBJ*
+	**bpftool struct_ops register** *OBJ* [*LINK_DIR*]
 		  Register bpf struct_ops from *OBJ*.  All struct_ops under
-		  the ELF section ".struct_ops" will be registered to
-		  its kernel subsystem.
+		  the ELF section ".struct_ops" and ".struct_ops.link" will
+		  be registered to its kernel subsystem.  For each
+		  struct_ops in the ".struct_ops.link" section, a link
+		  will be created.  You can give *LINK_DIR* to provide a
+		  directory path where these links will be pinned with the
+		  same name as their corresponding map name.
 
 	**bpftool struct_ops unregister**  *STRUCT_OPS_MAP*
 		  Unregister the *STRUCT_OPS_MAP* from the kernel subsystem.