This handler sets the `document` property of new annotations to metadata
extracted from `<meta>` and `<link>` tags on the page. It is unnecessary
however because the `Guest` class sets this property using either
`DocumentMeta.metadata` or `PDF.getMetadata`, depending on the document
type, before it emits the `beforeAnnotationCreated` event.

In PDFs this code is a serious hazard because if the
`beforeAnnotationCreated` event subscriber in `DocumentMeta` runs before
`AnnotationSync`'s corresponding event handler then DocumentMeta will
overwrite the PDF document-specific annotation metadata with generic
HTML document metadata - losing the critical `documentFingerprint`
property. Fortunately this problem did not occur because the `Guest`
constructor instantiates the `AnnotationSync` class before the
`DocumentMeta` class. Consequently AnnotationSync's
`beforeAnnotationCreated` event handler runs before DocumentMeta's and
pushes new annotations to the sidebar with the correct document metadata
before `DocumentMeta` overwrites it.