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/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