Update SurveyStack User Docs

Changes requiring an update of the docs (as soon as they're live)

• surveystack!136 (merged)
• surveystack#109 (closed)
• surveystack#126 (closed)
• surveystack!176 (merged)
• surveystack#125
• surveystack!201 (merged), also see summary in surveystack#15 (comment 1201225513)