bpftool: man: Add missing top level docs

The top-level (bpftool.8) man page was missing docs for a few
subcommands and their respective sub-sub-commands.

This commit brings the top level man page up to date. Note that I've
kept the ordering of the subcommands the same as in `bpftool help`.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/3049ef5dc509c0d1832f0a8b2dba2ccaad0af688.1647213551.git.dxu@dxuuu.xyz

Author: danobi

tools/bpf/bpftool/Documentation/bpftool.rst

@@ -20,7 +20,8 @@ SYNOPSIS
 
 	**bpftool** **version**
 
-	*OBJECT* := { **map** | **program** | **cgroup** | **perf** | **net** | **feature** }
+	*OBJECT* := { **map** | **program** | **link** | **cgroup** | **perf** | **net** | **feature** |
+	**btf** | **gen** | **struct_ops** | **iter** }
 
 	*OPTIONS* := { { **-V** | **--version** } | |COMMON_OPTIONS| }
 
@@ -31,6 +32,8 @@ SYNOPSIS
 	*PROG-COMMANDS* := { **show** | **list** | **dump jited** | **dump xlated** | **pin** |
 	**load** | **attach** | **detach** | **help** }
 
+	*LINK-COMMANDS* := { **show** | **list** | **pin** | **detach** | **help** }
+
 	*CGROUP-COMMANDS* := { **show** | **list** | **attach** | **detach** | **help** }
 
 	*PERF-COMMANDS* := { **show** | **list** | **help** }
@@ -39,6 +42,14 @@ SYNOPSIS
 
 	*FEATURE-COMMANDS* := { **probe** | **help** }
 
+	*BTF-COMMANDS* := { **show** | **list** | **dump** | **help** }
+
+	*GEN-COMMANDS* := { **object** | **skeleton** | **min_core_btf** | **help** }
+
+	*STRUCT-OPS-COMMANDS* := { **show** | **list** | **dump** | **register** | **unregister** | **help** }
+
+	*ITER-COMMANDS* := { **pin** | **help** }
+
 DESCRIPTION
 ===========
 	*bpftool* allows for inspection and simple modification of BPF objects