net: improve and fix netlink kdoc

Subsequent patch will render the kdoc from
include/uapi/linux/netlink.h into Documentation.
We need to fix the warnings. While at it move
the comments on struct nlmsghdr to a proper
kdoc comment.

Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

include/uapi/linux/netlink.h

···
        	__u32		nl_groups;	/* multicast groups mask */
 };
 
+/**
+ * struct nlmsghdr - fixed format metadata header of Netlink messages
+ * @nlmsg_len:   Length of message including header
+ * @nlmsg_type:  Message content type
+ * @nlmsg_flags: Additional flags
+ * @nlmsg_seq:   Sequence number
+ * @nlmsg_pid:   Sending process port ID
+ */
 struct nlmsghdr {
-	__u32		nlmsg_len;	/* Length of message including header */
-	__u16		nlmsg_type;	/* Message content */
-	__u16		nlmsg_flags;	/* Additional flags */
-	__u32		nlmsg_seq;	/* Sequence number */
-	__u32		nlmsg_pid;	/* Sending process port ID */
+	__u32		nlmsg_len;
+	__u16		nlmsg_type;
+	__u16		nlmsg_flags;
+	__u32		nlmsg_seq;
+	__u32		nlmsg_pid;
 };
 
 /* Flags values */
···
  *	bitfield32 type (U32)
  * @NL_POLICY_TYPE_ATTR_MASK: mask of valid bits for unsigned integers (U64)
  * @NL_POLICY_TYPE_ATTR_PAD: pad attribute for 64-bit alignment
+ *
+ * @__NL_POLICY_TYPE_ATTR_MAX: number of attributes
+ * @NL_POLICY_TYPE_ATTR_MAX: highest attribute number
  */
 enum netlink_policy_type_attr {
 	NL_POLICY_TYPE_ATTR_UNSPEC,