Added relativeCreated to Formatter doc (has been in the system for a long time...

Added relativeCreated to Formatter doc (has been in the system for a long time - was unaccountably left out of the docs and not noticed until now).

Doc/lib/liblogging.tex

@@ -1397,6 +1397,9 @@ Currently, the useful mapping keys in a \class{LogRecord} are:
                         (if available).}
 \lineii{\%(created)f}  {Time when the \class{LogRecord} was created (as
                         returned by \function{time.time()}).}
+\lineii{\%(relativeCreated)d}  {Time in milliseconds when the LogRecord was
+                        created, relative to the time the logging module was
+                        loaded.}
 \lineii{\%(asctime)s}  {Human-readable time when the \class{LogRecord}
                         was created.  By default this is of the form
                         ``2003-07-08 16:49:45,896'' (the numbers after the