Improve the descriptions about reference count (#62)

The term "reference count" would be better than "use count."
In addition, the usage of module_refcount(THIS_MODULE) was appended
for the reference counter query.

Author: fennecJ

lkmpg.tex

@@ -894,14 +894,15 @@ \subsection{Unregistering A Device}
 Normally, when you do not want to allow something, you return an error code (a negative number) from the function which is supposed to do it.
 With \cpp|cleanup_module| that's impossible because it is a void function.
 However, there is a counter which keeps track of how many processes are using your module.
-You can see what its value is by looking at the 3rd field of \verb|/proc/modules|.
+You can see what its value is by looking at the 3rd field with the command \sh|cat /proc/modules| or \sh|sudo lsmod|.
 If this number isn't zero, \sh|rmmod| will fail.
 Note that you do not have to check the counter within \cpp|cleanup_module| because the check will be performed for you by the system call \cpp|sys_delete_module|, defined in \src{include/linux/syscalls.h}.
 You should not use this counter directly, but there are functions defined in \src{include/linux/module.h} which let you increase, decrease and display this counter:
 
 \begin{itemize}
-  \item \cpp|try_module_get(THIS_MODULE)|: Increment the use count.
-  \item \cpp|module_put(THIS_MODULE)|: Decrement the use count.
+  \item \cpp|try_module_get(THIS_MODULE)|: Increment the reference count of current module.
+  \item \cpp|module_put(THIS_MODULE)|: Decrement the reference count of current module.
+  \item \cpp|module_refcount(THIS_MODULE)|: Return the value of reference count of current module.
 \end{itemize}
 
 It is important to keep the counter accurate; if you ever do lose track of the correct usage count, you will never be able to unload the module; it's now reboot time, boys and girls.