lib/modules: Add false positive info to check coherence assertion

Unfortunately, we don't have a good enough way to discern this
possibility. Technically we could add yet another attribute, but
that does not seem to be worth the cost at this point, in terms
of complexity and possibly even performance.

I take the blame for this one. I identified the need for the second
merge commit in review but didn't require it in the first merge.
These changes really should have been done in a single merge.
That would have prevented this situation.

lib/modules.nix

@@ -1147,6 +1147,9 @@ let
         instead of:
 
           baseType // { check = value: /* your check */; }
+
+        Alternatively, this message may also occur as false positive when mixing Nixpkgs
+        versions, if one Nixpkgs is between 83fed2e6..58696117 (Aug 28 - Oct 28 2025)
       '';
 
   # Merge definitions of a value of a given type.