tools: bpftool: doc, add text about feature-subcommand

This patch adds missing information about feature-subcommand in
bpftool.rst

Signed-off-by: Prashant Bhole <[e-mail]>
Reviewed-by: Quentin Monnet <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>

tools/bpf/bpftool/Documentation/bpftool.rst

···
 
 	**bpftool** **version**
 
-	*OBJECT* := { **map** | **program** | **cgroup** | **perf** | **net** }
+	*OBJECT* := { **map** | **program** | **cgroup** | **perf** | **net** | **feature** }
 
 	*OPTIONS* := { { **-V** | **--version** } | { **-h** | **--help** }
 	| { **-j** | **--json** } [{ **-p** | **--pretty** }] }
···
 	*PERF-COMMANDS* := { **show** | **list** | **help** }
 
 	*NET-COMMANDS* := { **show** | **list** | **help** }
+
+	*FEATURE-COMMANDS* := { **probe** | **help** }
 
 DESCRIPTION
 ===========