bpftool: Add bpftool-link manpage

Add bpftool-link manpage with information and examples of link-related
commands.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20200429001614.1544-10-andriin@fb.com

diff --git a/tools/bpf/bpftool/Documentation/bpftool-link.rst b/tools/bpf/bpftool/Documentation/bpftool-link.rst
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+================
+bpftool-link
+================
+-------------------------------------------------------------------------------
+tool for inspection and simple manipulation of eBPF links
+-------------------------------------------------------------------------------
+
+:Manual section: 8
+
+SYNOPSIS
+========
+
+       **bpftool** [*OPTIONS*] **link** *COMMAND*
+
+       *OPTIONS* := { { **-j** | **--json** } [{ **-p** | **--pretty** }] | { **-f** | **--bpffs** } }
+
+       *COMMANDS* := { **show** | **list** | **pin** | **help** }
+
+LINK COMMANDS
+=============
+
+|      **bpftool** **link { show | list }** [*LINK*]
+|      **bpftool** **link pin** *LINK* *FILE*
+|      **bpftool** **link help**
+|
+|      *LINK* := { **id** *LINK_ID* | **pinned** *FILE* }
+
+
+DESCRIPTION
+===========
+       **bpftool link { show | list }** [*LINK*]
+                 Show information about active links. If *LINK* is
+                 specified show information only about given link,
+                 otherwise list all links currently active on the system.
+
+                 Output will start with link ID followed by link type and
+                 zero or more named attributes, some of which depend on type
+                 of link.
+
+       **bpftool link pin** *LINK* *FILE*
+                 Pin link *LINK* as *FILE*.
+
+                 Note: *FILE* must be located in *bpffs* mount. It must not
+                 contain a dot character ('.'), which is reserved for future
+                 extensions of *bpffs*.
+
+       **bpftool link help**
+                 Print short help message.
+
+OPTIONS
+=======
+       -h, --help
+                 Print short generic help message (similar to **bpftool help**).
+
+       -V, --version
+                 Print version number (similar to **bpftool version**).
+
+       -j, --json
+                 Generate JSON output. For commands that cannot produce JSON, this
+                 option has no effect.
+
+       -p, --pretty
+                 Generate human-readable JSON output. Implies **-j**.
+
+       -f, --bpffs
+                 When showing BPF links, show file names of pinned
+                 links.
+
+       -n, --nomount
+                 Do not automatically attempt to mount any virtual file system
+                 (such as tracefs or BPF virtual file system) when necessary.
+
+       -d, --debug
+                 Print all logs available, even debug-level information. This
+                 includes logs from libbpf.
+
+EXAMPLES
+========
+**# bpftool link show**
+
+::
+
+    10: cgroup  prog 25
+            cgroup_id 614  attach_type egress
+
+**# bpftool --json --pretty link show**
+
+::
+
+    [{
+            "type": "cgroup",
+            "prog_id": 25,
+            "cgroup_id": 614,
+            "attach_type": "egress"
+        }
+    ]
+
+|
+| **# bpftool link pin id 10 /sys/fs/bpf/link**
+| **# ls -l /sys/fs/bpf/**
+
+::
+
+    -rw------- 1 root root 0 Apr 23 21:39 link
+
+
+SEE ALSO
+========
+       **bpf**\ (2),
+       **bpf-helpers**\ (7),
+       **bpftool**\ (8),
+       **bpftool-prog\ (8),
+       **bpftool-map**\ (8),
+       **bpftool-cgroup**\ (8),
+       **bpftool-feature**\ (8),
+       **bpftool-net**\ (8),
+       **bpftool-perf**\ (8),
+       **bpftool-btf**\ (8)