doc, explain new reference year for daisy dates

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
doc, leave a note about the Gregorian Reformation assumption in dateutils

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
doc, mention new reference date in format.texi as well

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
Merge branch 'doc/general' into next

* doc/general:
  doc, mention new reference date in format.texi as well
  doc, leave a note about the Gregorian Reformation assumption in dateutils
  doc, explain new reference year for daisy dates