Expand and reframe discussion of optionals (#143)

- Reframe the discussion of optional unwrapping to not conceptually rely
  on force unwrapping.
- Highlight the value of non-optional types *never* being nil, and how
  you can't forget to handle nil on an optional.
- Explicitly list the ways you can handle a `nil` value, and briefly
  introduce the `??` operator.
- Explain when you should consider using force unwrapping.
- Reduce the comparison to C and Objective-C.
- Fix some mis-use of note boxes, where they were highlighting
  information instead of asides that readers can choose to skip.

Fixes: rdar://41014749

Author: amartini51

Style.md

@@ -171,6 +171,10 @@ For example:
 If the operator doesn’t have an established English name,
 make sure you get tech review on the name you invent for it.
 
+## optional binding
+
+Omit the article: "use optional binding", not "use an optional binding".
+
 ## passive voice
 
 In general,