Add desc for orders element clarify what fhir resources

[isaacvetter]

Fixes: https://jira.hl7.org/browse/FHIR-42365
and https://jira.hl7.org/browse/FHIR-42678

I considered adding the same blurb to order-dispatch, but I don't think it really needs it.

[isaacvetter]

Also adding a railroad diagram b/c https://jira.hl7.org/browse/FHIR-42364

Diagram(
  NonTerminal("Open CPOE"),
  Group(Choice(0,NonTerminal("Select product"),"order-select"),"order-select fires at the beginning"),
  Choice(1, 
    NonTerminal("Select route"),
    NonTerminal("Select frequency"),
    NonTerminal("Select duration"),
    NonTerminal("Link Order & misc"),
  ),
  Group(Choice(0,NonTerminal("Sign order"),"order-sign"),"order-sign at the end")
  )

here: https://tabatkins.github.io/railroad-diagrams/generator.html#Diagram(%0A%20%20NonTerminal(%22Open%20CPOE%22)%2C%0A%20%20Group(Choice(0%2CNonTerminal(%22Select%20product%22)%2C%22order-select%22)%2C%22order-select%20fires%20at%20the%20beginning%22)%2C%0A%20%20Choice(1%2C%20%0A%20%20%20%20NonTerminal(%22Select%20route%22)%2C%0A%20%20%20%20NonTerminal(%22Select%20frequency%22)%2C%0A%20%20%20%20NonTerminal(%22Select%20duration%22)%2C%0A%20%20%20%20NonTerminal(%22Link%20Order%20%26%20misc%22)%2C%0A%20%20)%2C%0A%20%20Group(Choice(0%2CNonTerminal(%22Sign%20order%22)%2C%22order-sign%22)%2C%22order-sign%20at%20the%20end%22)%0A%20%20)