docs(io): state when an empty clamp sheet is omitted

The empty-plate rule drops a rail-following empty sheet only while it
still matches the plate defaults; a trailing sheet created before the
defaults changed is written. Say so instead of promising that the
editor's trailing sheet is always dropped. Comment and documentation
only; behaviour unchanged.

Delta review minor finding, Project Memory 4e5f566d.

OpenNest.IO/NestWriter.cs

@@ -228,7 +228,8 @@ namespace OpenNest.IO
                 var plate = nest.Plates[i];
 
                 // An empty plate is omitted unless that would lose a clamp configuration the
-                // saved defaults cannot recreate; the editor's trailing empty sheet is dropped.
+                // saved defaults cannot recreate; a trailing empty sheet that still matches
+                // the defaults is dropped.
                 if (
                     plate.Parts.Count(p => !p.BaseDrawing.IsCutOff) == 0
                     && plate.CutOffs.Count == 0

docs/nest-file-format.md

@@ -83,8 +83,9 @@ An empty plate (no parts, no cut-offs) is omitted from the file unless that
 would lose clamp zones: it is omitted like any other empty plate only when a
 new plate from the saved defaults would recreate it (same size and quadrant,
 zones the saved rail recreates), which keeps the editor's trailing empty sheet
-out of the file. An empty plate with other zones, or a rail-following one
-whose size or quadrant differs from the defaults, is written.
+out of the file while it still matches the defaults. An empty plate with other
+zones, or a rail-following one whose size or quadrant differs from the defaults
+(including a trailing sheet created before the defaults changed), is written.
 
 ## Drawing edits