[docs]: Replace deprecated subscribe() methods in documentation

[jakovljevic-mladen]

Documentation Related To Component:

Observable

Please check those that apply

• typo
• documentation doesn't exist
• documentation needs clarification
• error(s) in example
• needs example
• needs update

Description Of The Issue

Observable's subscribe() method has some examples how to pass parameters, but there are some issues there:

• Subscribe with an Observer example is probably fine.
• Subscribe with functions uses deprecated subscribe method. Should a deprecated example be in the docs? It's OK to show users all (or many) possible ways to subscribe, but I think that deprecated methods should be noted or that examples with deprecated methods should not be used. This example (of summing 1, 2 and 3) is probably bad since it waits for complete event and having complete handler is not deprecated only if used with observer. I'd suggest changing an example (only to use next handler), but I'd add a note that adding other handlers should be probably done by passing an observer object. Is that OK?
• Cancel a subscription example should use observer as parameter with no error handler at all.