DOC Document added generic types

[GuySartorelli]

Description

Documents the fact that we're adding generic typehints, what the intention is, and changes that developers can make to take full advantage of the new typehints.

Has two commits - the first is a small change to linting rules to allow use statements that are only used in annotations. This is required for the @extends Extension<SiteConfig> example.

Issues

• Add generic typehints in PHPDoc silverstripe-framework#11066

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[emteknetnz]

Mention the code that was deprecated in the framework PR

[GuySartorelli]

Done (and rebased)