bpf: Document BPF_PROG_ATTACH syscall command

Document the prog attach command in more detail, based on git commits:
* commit f4324551 ("bpf: add BPF_PROG_ATTACH and BPF_PROG_DETACH
  commands")
* commit 4f738adb ("bpf: create tcp_bpf_ulp allowing BPF to monitor
  socket TX/RX data")
* commit f4364dcf ("media: rc: introduce BPF_PROG_LIRC_MODE2")
* commit d58e468b ("flow_dissector: implements flow dissector BPF
  hook")
Signed-off-by: Joe Stringer <joe@cilium.io>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210302171947.2268128-6-joe@cilium.io

include/uapi/linux/bpf.h

@@ -253,6 +253,43 @@ union bpf_iter_link_info {
  *		Attach an eBPF program to a *target_fd* at the specified
  *		*attach_type* hook.
  *
+ *		The *attach_type* specifies the eBPF attachment point to
+ *		attach the program to, and must be one of *bpf_attach_type*
+ *		(see below).
+ *
+ *		The *attach_bpf_fd* must be a valid file descriptor for a
+ *		loaded eBPF program of a cgroup, flow dissector, LIRC, sockmap
+ *		or sock_ops type corresponding to the specified *attach_type*.
+ *
+ *		The *target_fd* must be a valid file descriptor for a kernel
+ *		object which depends on the attach type of *attach_bpf_fd*:
+ *
+ *		**BPF_PROG_TYPE_CGROUP_DEVICE**,
+ *		**BPF_PROG_TYPE_CGROUP_SKB**,
+ *		**BPF_PROG_TYPE_CGROUP_SOCK**,
+ *		**BPF_PROG_TYPE_CGROUP_SOCK_ADDR**,
+ *		**BPF_PROG_TYPE_CGROUP_SOCKOPT**,
+ *		**BPF_PROG_TYPE_CGROUP_SYSCTL**,
+ *		**BPF_PROG_TYPE_SOCK_OPS**
+ *
+ *			Control Group v2 hierarchy with the eBPF controller
+ *			enabled. Requires the kernel to be compiled with
+ *			**CONFIG_CGROUP_BPF**.
+ *
+ *		**BPF_PROG_TYPE_FLOW_DISSECTOR**
+ *
+ *			Network namespace (eg /proc/self/ns/net).
+ *
+ *		**BPF_PROG_TYPE_LIRC_MODE2**
+ *
+ *			LIRC device path (eg /dev/lircN). Requires the kernel
+ *			to be compiled with **CONFIG_BPF_LIRC_MODE2**.
+ *
+ *		**BPF_PROG_TYPE_SK_SKB**,
+ *		**BPF_PROG_TYPE_SK_MSG**
+ *
+ *			eBPF map of socket type (eg **BPF_MAP_TYPE_SOCKHASH**).
+ *
  *	Return
  *		Returns zero on success. On error, -1 is returned and *errno*
  *		is set appropriately.