[Validator] Consistently use Assert\... when referring to assertions

[mpdude]

I noticed a slight inconsistency in the docs when showing examples of using Constraint classes. In some examples, the class is used and shown with the short class name. In other cases, the Symfony\Component\Validator\Constraints namespace is used as Assert, and references being made as Assert\NotBlank and similar.

Personally, I like the Assert\ prefix, since it triggers the "ah, that's a constraint" wiring in my head, but I don't mind. Generally, I'd prefer consistency, since I noticed that this caused confusion in my team.

Additionally, this PR fixes a few places where we are still showing the use of array-based options instead of named arguments, a change that was made in 7.3.

[mpdude]

Seems the code block checker does not support namespace-based imports, only direct class references?

[javiereguiluz]

Thanks Matthias. I like this because, as you said, most examples were already using Assert, so let's be consistent!