bpftool: add missing fsession to the usage and docs of bpftool

bpftool: add missing fsession to the usage and docs of bpftool

Add the fsession attach type to the usage of bpftool in do_help().
Meanwhile, add it to the bash-completion and bpftool-prog.rst too.

Acked-by: Leon Hwang <leon.hwang@linux.dev>
Acked-by: Quentin Monnet <qmo@kernel.org>
Signed-off-by: Menglong Dong <dongml2@chinatelecom.cn>
Link: https://lore.kernel.org/r/20260412060346.142007-4-dongml2@chinatelecom.cn
Signed-off-by: Alexei Starovoitov <ast@kernel.org>