solid.mirror: a plane's {origin} takes length expressions and follows them (#1492) (#1501)

Thanks @erkexzcx.

crates/doc/src/format/pending/placed-plane-origins.rs

@@ -0,0 +1,4 @@
+//! An explicit plane (a mirror's, or any plane reference) can be `placed`: an `origin` of three
+//! length expressions with `x_dir` and `y_dir`, so it follows the design's parameters (#1492). A
+//! plane whose origin is plain numbers is still saved as `custom`. Older designs need no
+//! transformation; 0.13.x asks for an update instead of failing to read the plane.