Include documentation for `assertComplete` and `assertFailure` methods

[ThorodanBrom]

Describe the feature

• Since Vert.x 5.0 is deprecating the callback model, I was wondering if the callbacks used in methods like succeeding and failing would stop working in 5.0 (or maybe I have not understood the deprecation properly)
• I found out about the existence of the methods assertComplete and assertFailed developed by @slinkydeveloper from their blog, which allow for testing with the help of Futures instead of the callbacks
• However, there isn't any official documentation about these methods in the official docs

Use cases

• Including documentation for this may help those who are looking to move away from Handler<AsyncResult> -style callbacks in JUnit tests
• If succeeding, failing are getting deprecated, then assertComplete and assertFailing would be the only methods usable (I think I am wrong here)

Contribution

• I can try to add documentation based on @slinkydeveloper's blog post

[tsegismont]

No, these methods won't stop working, it's perfectly fine to use them, for example, at the the end of a future composition chain.

Note that in Vert.x 5 you can create Future expectations with Future.expecting

It would be great if you could contribute a doc update about test context methods related to futures.