Improve file operation intro

Done while answering: https://stackoverflow.com/questions/3290590/what-options-do-we-have-for-communication-between-a-user-program-and-a-linux-ker/51206012#51206012

Author: cirosantilli

README.adoc

@@ -3687,12 +3687,13 @@ Sources:
 
 Bibliography: https://stackoverflow.com/questions/5970595/how-to-create-a-device-node-from-the-init-module-code-of-a-linux-kernel-module/45531867#45531867
 
-
 === Pseudo files
 
 ==== File operations
 
-File operations are the main method of userland driver communication. `struct file_operations` determines what the kernel will do on filesystem system calls of <<pseudo-filesystems>>:
+File operations are the main method of userland driver communication. `struct file_operations` determines what the kernel will do on filesystem system calls of <<pseudo-filesystems>>.
+
+This example illustrates the most basic system calls: `open`, `read`, `write`, `close` and `lseek`:
 
 ....
 /fops.sh
@@ -3919,7 +3920,7 @@ This example gets an anonymous inode via <<ioctl>> from a debugfs entry by using
 
 Reads to that inode return the sequence: `1`, `10`, `100`, ... `10000000`, `1`, `100`, ...
 
-Bibliography: https://stackoverflow.com/questions/4508998/what-is-an-anonymous-inode-in-linux
+Bibliography: https://stackoverflow.com/questions/4508998/what-is-an-anonymous-inode-in-linux/44388030#44388030
 
 ==== netlink sockets