Meta: Document the API

[zploskey]

We are missing doc comments for a lot of the bindings. Adding them really improves the developer experience in the editor and will eventually allow us to generate some API documentation. We'll check off documented modules as they are done.

• BoundingBox
• BoxModel
• Browser
• BrowserContext
• BrowserFetcher
• CDPSession
• Click
• ConsoleMessage
• Coverage
• Dialog
• ElementHandle
• Evaluator
• ExecutionContext
• Frame
• FrameBase
• JSHandle
• JSMap (may eventually be split off into a separate package)
• Keyboard
• Metrics
• Mouse
• Navigation
• Page
• Puppeteer (partially documented)
• Request (partially documented)
• Response
• Screenshot
• SecurityDetails
• Target
• Touchscreen (and some more binding implementation)
• Tracing
• Types
• Unit
• Util
• Worker

For now we should focus on getting the most important information into doc comments on the main functions and types of each modules. Don't worry too much about formatting until things have standardized somewhat. Short doc comments may need to be padded out some until Reason 3.3+ is released with my fix to doc comment formatting before externals (reasonml/reason@1502999).