Adopt the phpdoc standard for documenting collection (eg Iterator) key and value types

Problem/Motivation

In #3161129: [PP-1] Fix phpdoc return type for RouteProvider::getAllRoutes() we're trying to document the @return value of a function that returns an \Iterator. For good reason, we like to be specific. Apparently, phpdoc now supports a syntax for defining the type of the values (and optionally, keys) in an an array or Iterator. It's something like this:

@return \Iterator<string, \Symfony\Component\Routing\Route>

However, we don't currently do this anywhere, and Coder doesn't understand this syntax yet (see #3347758: [PP-1] Support the recommended phpdoc syntax for documenting Iterator return types).

So we need to discuss if this is the right thing, decide on the appropriate style for it (e.g. do we put the space after the comma?), etc.

Steps to reproduce

Proposed resolution

When documenting collections (eg Iterator and friends), allow using the <keyType, valueType> syntax to define the types of the keys and values.

If the keys don't matter, you can just use the valueType like so:
<valueType>

Remaining tasks

User interface changes

API changes

Data model changes