Add a bit to the WEEKS comment

Change-Id: I8d409149710d25aa93e33f3c46723239156f4d1e
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/126304
Tested-by: Jenkins
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>

scaddins/source/datefunc/datefunc.cxx

@@ -455,7 +455,8 @@ sal_Int32 GetNullDate( const uno::Reference< beans::XPropertySet >& xOptions )
  *
  * The (consecutive) week number of a date is
  * std::floor( (date + NullDate - 1), 7.0 ),
- * with weeks starting on Monday.
+ * with weeks starting on Monday, and week 0
+ * starting on Monday, 0001-01-01 Gregorian.
  *
  * Weeks(d2,d1,m) is defined as -Weeks(d1,d2,m).
  *