face.delete heal: false leaves an open surface body (#861)

Delete Face takes heal (default true, as before). With heal: false the picked faces are removed and the gap stays open, leaving the remaining faces as a surface body (kernel remove_faces); deleting every face is refused. Saved only when false, noted under format 18.

crates/doc/src/format.rs

@@ -113,6 +113,9 @@
 //!   `letter_spacing` (mm), `line_spacing` (times the font's line distance), `path` (the id of a
 //!   curve of the same sketch the baseline follows) and `reverse`. A text without `style` is set
 //!   as it always was; 0.9.0 would drop the style and set the text plain, off its curve.
+//!   Under the same version, Delete Face can leave the gap open (`heal: false` on the feature:
+//!   the faces left are a surface body); a Delete Face without it heals, as it always did, and
+//!   saves as before. 0.9.0 would heal a gap the design leaves open.
 
 use serde_json::Value;