Though it may have started as an unofficial document, it is clearly now
an official one (which should be obvious since it is in our source
repository, but apparently some people get misled by the historical
"Status" text to think this to be somehow unofficial).
So first of all, change the s/official/unofficial/ mention.

Secondly, add a small paragraph explicitly telling that the document is
complete (and meant to be), to the best of our knowledge. This document
is a detailed, full and exhaustive written "specification" of the XCF
format up to GIMP 2.10.x (even though the normative spec is still the
code itself). Now we are humans, we may have missed something, and if
so, this is just to be considered as any other bug, and reported to us
nicely to be fixed.