Generalize span creation (open-telemetry#190)

* Revise span creation, scrub references to builder

* Make it clear that root spans aren't optional

* Move attrs, reword multiple sections

* Lose StartSpan

* Update active span description for PR comments

* Note that span attrs are ordered

* Summarize span config, add details

* Apply @arminru's suggestions from code review

Co-Authored-By: Armin Ruech <[email protected]>

* Add notes on attribute, event, link ordering

* Clarify optional/required span fields

* Remove attribute ordering note