(SRFI-19): Note Gregorian leap year rules

incorrectly extended back prior to 1582.

doc/ref/srfi-modules.texi

@@ -1632,6 +1632,17 @@ functions and variables described here are provided by
 (use-modules (srfi srfi-19))
 @end example
 
+@strong{Caution}: The current code in this module incorrectly extends
+the Gregorian calendar leap year rule back prior to the introduction
+of those reforms in 1582 (or the appropriate year in various
+countries).  The Julian calendar was used prior to 1582, and there
+were 10 days skipped for the reform, but the code doesn't implement
+that.
+
+This will be fixed some time.  Until then calculations for 1583
+onwards are correct, but prior to that any day/month/year and day of
+the week calculations are wrong.
+
 @menu
 * SRFI-19 Introduction::        
 * SRFI-19 Time::