Review and improve the JavaScript documentation

[egamma]

Cover:

• what is the difference between a project that doesn't have a jsconfig and one that has one
• how to add a .d.ts by either using a /// reference or by adding a jsconfig.json that containts the typings folder
• what are the semantics of the compiler options section
  • target - defines which libraries are available
• how to disable the built-in JS validation when you want to use ES7 constructs
• reload JavaScript project (the good news is that this no longer has to be used for normal flows)
• Document how to get Intellisense for ReactNative