fix: add beforeDocumentControls to globals generate importmap

[rilrom]

What?

When adding beforeDocumentControls as a custom component to a global, it does not get picked up by the import map, even when running generate:importmap.

Why?

beforeDocumentControls is missing from addToImportMap in iterateGlobals.

How?

Adds beforeDocumentControls to addToImportMap in iterateGlobals.

While doing this I've used the opportunity to add Description to the globals custom component documentation as it was missing.

Fixes #15035