Adding code comments and @hidden tags for src/providers/database.ts

[esmacik]

• Adding code comments to database.ts for database.ref() method, database.RefBuilder and database.DataSnapshot from existing database-externs.js file
• Adding @hidden tags to any variable or method with an underscore before it in database.DataSnapshot

[esmacik]

Here's also a link to see the staged changes 😄 https://firebase.devsite.corp.google.com/docs/reference/functions/providers_database_.html

[thechenky]

Looks great, thanks!