docs: start documenting lazy-pages

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

Documentation/criu.txt

@@ -317,6 +317,15 @@ For example, the command line for the above example should look like this:
     engine's default directory for tasks, permissions will not be preserved on
     the upper directories with no tasks in them, which may cause problems.
 
+*--lazy-pages*::
+    Perform the dump procedure without writing memory pages into the
+    image files and prepare to service page requests over the
+    network. When *dump* runs in this mode it presumes that
+    *lazy-pages* daemon will connect to it and fetch memory pages to
+    lazily inject them into the restored process address space. This
+    option is intended for post-copy (lazy) migration and should be
+    used in conjunction with *restore* with appropriate options.
+
 *restore*
 ~~~~~~~~~
 Restores previously checkpointed processes.
@@ -466,6 +475,14 @@ are not adequate, but this can be suppressed by using *--cpu-cap=none*.
    to restore on an older kernel, or a kernel configured without some
    options.
 
+*--lazy-pages*::
+   Restore the processes without filling out the entire memory
+   contents. When this option is used, *restore* sets up the
+   infrastucture required to fill memory pages either on demand when
+   the process accesses them or in the background without stopping the
+   restored process.
+   This option requires running *lazy-pages* daemon.
+
 *check*
 ~~~~~~~
 Checks whether the kernel supports the features needed by *criu* to
@@ -532,6 +549,18 @@ Launches *criu* in page server mode.
 *--port* 'number'::
     Page server port number.
 
+*lazy-pages*
+~~~~~~~~~~~~
+Launches *criu* in lazy-pages daemon mode.
+
+The *lazy-pages* daemon is responsible for managing user-level demand
+paging for the restored processes. It gets information required to
+fill the process memory pages from the *restore* and from the
+checkpont directory. When a restored process access certain memory
+page for the first time, the *lazy-pages* daemon injects its contents
+into the process address space. The memory pages that are not yet
+requested by the restored processes are injected in the background.
+
 *exec*
 ~~~~~~
 Executes a system call inside a destination task\'s context. This functionality