tools/bpftool: Add documentation and sample output for process info

Add statements about bpftool being able to discover process info, holding
reference to BPF map, prog, link, or BTF. Show example output as well.

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/20200619231703.738941-10-andriin@fb.com

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

@@ -62,6 +62,11 @@ DESCRIPTION
 		  Output will start with map ID followed by map type and
 		  zero or more named attributes (depending on kernel version).
 
+		  Since Linux 5.8 bpftool is able to discover information about
+		  processes that hold open file descriptors (FDs) against BPF
+		  maps. On such kernels bpftool will automatically emit this
+		  information as well.
+
 	**bpftool map create** *FILE* **type** *TYPE* **key** *KEY_SIZE* **value** *VALUE_SIZE*  **entries** *MAX_ENTRIES* **name** *NAME* [**flags** *FLAGS*] [**dev** *NAME*]
 		  Create a new map with given parameters and pin it to *bpffs*
 		  as *FILE*.
@@ -180,7 +185,8 @@ EXAMPLES
 ::
 
   10: hash  name some_map  flags 0x0
-	key 4B  value 8B  max_entries 2048  memlock 167936B
+        key 4B  value 8B  max_entries 2048  memlock 167936B
+        pids systemd(1)
 
 The following three commands are equivalent: