[QUEST] Fix Gaps in Documentation

[anlumo]

As requested by @runspired, here are the things I found to be undocumented, but required to be known for using ember-data:

• [x] snapshot, as passed into many DS.Adapter functions.
• [ ] type, as passed into many DS.Adapter functions.
• [ ] The meta object given to the function passed to the functions DS.Model.eachRelationship and DS.Model.eachAttribute.
• [ ] How to notify the store about a record deletion from the server side.
• [ ] DS.belongsTo and DS.hasMany being async by default.
• [ ] When to use unloadRecord.
• [ ] How to unset attributes in a way that they revert back to the defaultValue instead of null.
• [x] A full step-by-step guide on how to implement the whole ember-data layer with a custom adapter that does not inherit from DS.JSONAdapter (including serializers).
• [ ] The undocumented fact that you have to push all changes that happened locally back to the store once they're on the server

Related: #4375

[runspired]

@anlumo working on getting lots of doc fixes resource, just wanted to say thanks for opening this ♥️

[runspired]

Going to roll this into #8394