Update Input Docs

[StephenHodgson]

Update all Input Documentation with recent changes:

• Updated uGUI workflow
• Raycasting and using layer masks. See EventSystem - what is UIRaycastCamera? #1442
• Update interface list and scripts with public properties listed
• Sample Input Implementation with a list of best practices
  • Advise devs to always consume event data via eventData.Use()
• How to extend the InputManager with custom input sources, event data, and handlers
• Update Prefab definitions
• Update Script definitions
• Update all test definitions

(please add suggestions below)

[StephenHodgson]

Here's a working copy of the improved Input documentation I'm writing if anyone is interested