doc: Add syscalls execution description

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

Documentation/crtools.txt

@@ -31,6 +31,9 @@ OPTIONS
         * 'check'
                 to test whether the kernel support is up-to-date
 
+	* 'exec'
+		to execute a system call from other task's context
+
 -c::
     In case of 'show' command the dumped pages content will be shown in hex format.
 
@@ -109,6 +112,27 @@ OPTIONS
 -h|--help::
     Print inline help.
 
+SYSCALLS EXECUTION
+------------------
+
+To run a system call from another task's context use
+
+    crtools exec -t <pid> <syscall-string>
+
+command. The syscall string should look like
+
+    <syscall-name> <syscall-arguments>...
+
+Each command line argument is transformed into the system call argument by
+the following rules:
+
+* If one starts with '&', the rest of it gets copied to the target task's
+  address space and the respective syscall argument is the pointer to this
+  string;
+
+* Otherwise it's treated as a number (converted with strtol) and is directly
+  passed into the system call.
+
 EXAMPLES
 --------
 
@@ -121,6 +145,17 @@ To restore this program detaching crtools itself, one should type
 
     crtools restore -d -D checkpoint -t 1234
 
+
+To close a file descriptor number '1' in task with pid '1234' run
+
+    crtools exec -t 1234 close 1
+
+To open a file named '/foo/bar' for read-write in the task with pid
+'1234' run
+
+    crtools exec -t 1234 open '&/foo/bar' 2
+
+
 AUTHOR
 ------
 OpenVZ team.