doc/api-sessions (make-session): Improve the description

* doc/api-sessions.texi (make-session): Improve the description.

Author: artyom-poptsov

doc/api-sessions.texi

@@ -80,7 +80,8 @@ Example:
 @end lisp
 
 All options are described in detail in @code{session-set!} procedure
-description.
+description.  Options that are read from @var{config} file will be overwritten
+by the values passed to @code{make-session}.
 
 List of allowed keywords:
 @table @samp