Add clarifying info on non-destructive behavior for update()

[DavidFrahm]

Version info

Angular:
4.1.3

Firebase:
4.3.0

AngularFire:
4.0.0-rc.2

Other (e.g. Ionic/Cordova, Node, browser, operating system):

How to reproduce these conditions

Failing test unit, Plunkr, or JSFiddle demonstrating the problem

Current docs for Saving Data do not clarify that non-destructive behavior for update() only applies to values under the key level:

• https://github.com/angular/angularfire2/blob/master/docs/2-retrieving-data-as-objects.md#saving-data

Steps to set up and reproduce

Sample data and security rules

<-- include/attach/link to some json sample data (or provide credentials to a sanitized, test Firebase project) -->

Debug output

** Errors in the JavaScript console **

** Output from firebase.database().enableLogging(true); **

** Screenshots **

Expected behavior

That the docs be updated slightly to reflect the implementation details of updating with objects regarding destructive vs non-destructive.

Actual behavior

Because this detail is currently not provided, users tend to think all properties nested within objects being updated should be non-destructive. This leads to unexpected behavior and confusion.