"EXIF schema for additional EXIF properties" silently disappeared since XMP Specification Part 2 - 2016

Until "XMP Specification Part 2 - Additional Properties" revision of 2008, `aux:Lens`, `aux:SerialNumber` existed in the `aux` namespace, see the following excerpt:

Possibly they may have been replaced by `exifEX:LensModel` and `exifEX:LensSerialNumber` in [CIPA DC-010-2012](https://www.cipa.jp/std/documents/download_e.html?DC-010-2012_E) (the separated document specifying all of the Exif properties since XMP Specification Part 2 of 2016), however this is not documented either in the current specification [pdf document](https://github.com/adobe/XMP-Toolkit-SDK/blob/main/docs/XMPSpecificationPart2.pdf) or the [Markdown](https://github.com/adobe/xmp-docs/tree/master/XMPNamespaces) rendition.

I believe the two properties should be either fully reinstated in "XMP Specification Part 2" or reinstated as deprecated.

This issue has been first created in the [pdf-issues](https://github.com/pdf-association/pdf-issues/issues/620) repository but moved to this one after the PDF TWG discussion.

Compare the current XMPSpecificationPart2.pdf with the Markdown files in XMPNamespaces, focusing on aux:Lens, aux:SerialNumber, exifEX:LensModel, and exifEX:LensSerialNumber. Read the linked CIPA DC-010-2012 reference and the discussion history from pdf-issues #620. Done means the documentation clearly records whether the properties were replaced, reinstated, or deprecated in both renditions.

Written by the indexing model from the issue text.