Angular needs to improve deprecation process

[yjbanov]

Currently when something is deprecated in Angular that thing is annotated with @deprecated. And that's it! Unfortunately, this does not tell the user how to proceed. Since this happened several times already, I'm logging this so a better deprecation process is documented in the coding guidelines and followed by contributors.

We should start with:

• Each @deprecated annotation must be accompanied by a paragraph explaining where the users should go and also how long users have until the deprecated thing goes away (preferably in a form of a date, e.g. 2014-06-27).
• When something is deprecated, the behavior must not change

[vicb]

👍
We should also use @Depreacted(msg)

Where msg informs about when it will be deprecated and how o upgrade