doc: add documentation for date modifiers in eve-log

doc/userguide/output/eve/eve-json-output.rst

@@ -203,6 +203,21 @@ enabled, then the log gets more verbose.
 
 By using ``custom`` it is possible to select which TLS fields to log.
 
+Date modifiers in filename
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+It is possible to use date modifiers in the eve-log filename.
+
+::
+
+   outputs:
+     - eve-log:
+         filename: eve-%s.json
+
+The example above adds epoch time to the filename. All the date modifiers from the
+C library should be supported. See the man page for ``strftime`` for all supported
+modifiers.
+
 Rotate log file
 ~~~~~~~~~~~~~~~