tools/bpftool: Add documentation and bash-completion for `link detach`

Add info on link detach sub-command to man page. Add detach to bash-completion
as well.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Acked-by: John Fastabend <john.fastabend@gmail.com.
Link: https://lore.kernel.org/bpf/20200731182830.286260-6-andriin@fb.com

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

@@ -21,6 +21,7 @@ LINK COMMANDS
 
 |	**bpftool** **link { show | list }** [*LINK*]
 |	**bpftool** **link pin** *LINK* *FILE*
+|	**bpftool** **link detach *LINK*
 |	**bpftool** **link help**
 |
 |	*LINK* := { **id** *LINK_ID* | **pinned** *FILE* }
@@ -49,6 +50,13 @@ DESCRIPTION
 		  contain a dot character ('.'), which is reserved for future
 		  extensions of *bpffs*.
 
+	**bpftool link detach** *LINK*
+		  Force-detach link *LINK*. BPF link and its underlying BPF
+		  program will stay valid, but they will be detached from the
+		  respective BPF hook and BPF link will transition into
+		  a defunct state until last open file descriptor for that
+		  link is closed.
+
 	**bpftool link help**
 		  Print short help message.
 

tools/bpf/bpftool/bash-completion/bpftool

@@ -1122,7 +1122,7 @@ _bpftool()
             ;;
         link)
             case $command in
-                show|list|pin)
+                show|list|pin|detach)
                     case $prev in
                         id)
                             _bpftool_get_link_ids
@@ -1139,7 +1139,7 @@ _bpftool()
                     COMPREPLY=( $( compgen -W "$LINK_TYPE" -- "$cur" ) )
                     return 0
                     ;;
-                pin)
+                pin|detach)
                     if [[ $prev == "$command" ]]; then
                         COMPREPLY=( $( compgen -W "$LINK_TYPE" -- "$cur" ) )
                     else