Documentation: update RPC configuration file section

Signed-off-by: Adrian Reber <areber@redhat.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

Documentation/criu.txt

@@ -609,12 +609,25 @@ they are compatible with the ones present in an image file.
 
 CONFIGURATION FILES
 -------------------
-Criu supports usage of configuration files to avoid the need of writing every
+*Criu* supports usage of configuration files to avoid the need of writing every
 option on command line, which is useful especially with repeated usage of
 same options. A specific configuration file can be passed with
-"*--config* 'file'" option. If no file is passed, default configuration files
-'/etc/criu/default.conf' and '$HOME/.criu/default.conf' are parsed (if
-present on the system). Default configuration file parsing can be forbidden
+the "*--config* 'file'" option. If no file is passed, the default configuration
+files '/etc/criu/default.conf' and '$HOME/.criu/default.conf' are parsed (if
+present on the system). If the environment variable CRIU_CONFIG_FILE is set,
+it will also be parsed.
+
+The options parsed to CRIU via CLI, RPC or configuration file are evaluated
+in the following order:
+
+    - apply_config(/etc/criu/default.conf)
+    - apply_config($HOME/.criu/default.conf)
+    - apply_config(CRIU_CONFIG_FILE)
+    - apply_config(*--config* 'file')
+    - apply_config(CLI) or apply_config(RPC)
+    - apply_config(RPC configuration file) (only for RPC mode)
+
+Default configuration file parsing can be deactivated
 with "*--no-default-config*" if needed. Parsed configuration files are merged
 with command line options, which allows overriding boolean options.
 
@@ -638,6 +651,23 @@ work-dir "/home/USERNAME/criu/my \"work\" directory"
 no-restore-sibling   # this is another comment
 ---------------
 
+Configuration files in RPC mode
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Not only does *criu* evaluate configuration files in CLI mode, it also
+evaluates configuration files in RPC mode. Just as in CLI mode the
+configuration file values are evaluated first. This means that any option
+set via RPC will overwrite the configuration file setting. The user can
+thus change *criu*'s default behavior but it is not possible to change
+settings which are explicitly set by the RPC client.
+
+The RPC client can, however, specify an additional configuration file
+which will be evaluated after the RPC options (see above for option evaluation
+order). The RPC client can specify this additional configuration file
+via "req.opts.config_file = '/path/to/file'". The values from this
+configuration file will overwrite all other configuration file settings
+or RPC options. *This can lead to undesired behavior of criu and
+should only be used carefully.*
+
 EXAMPLES
 --------
 To checkpoint a program with pid of *1234* and write all image files into