Documentation for switch statements with no control expression

Added the following to the docs in the Switch section:

Switch statements can also be used without a control expression, turning them in to a cleaner alternative to if/else chains.

score = 76
grade = switch
  when score < 60 then 'F'
  when score < 70 then 'D'
  when score < 80 then 'C'
  when score < 90 then 'B'
  else 'A'
#grade == 'C'