doc, rephrase Calendars section
doc, move section ywd further atop
doc, mention lilian/ldn and julian/jdn in Calendars section

Also, emphasize the designators again in the Formats section.
Merge branch 'doc/calendars'

* doc/calendars:
  doc, mention lilian/ldn and julian/jdn in Calendars section
  doc, move section ywd further atop
  doc, rephrase Calendars section