So, not "last built", "last updated" which is when the source file was last updated (time of last commit?).

The idea here is to give the reader some information about when the document was last updated by a human. That quickly tells us how old the information on the page is, and that tells the reader a little about whether the information is up to date or not.

This may not be required for release notes, but things like quick docs could benefit from this.