Document shared source version checks

layered.go

@@ -25,7 +25,9 @@ type Underlay struct{ Inputs []Input }
 // ignores it. Overrides remain explicit CLI records, not general mappings.
 // SourceVersionMode applies one declaration policy to main, underlay, import,
 // and selected overlay documents. Enabled modes interpret the version path
-// source-locally while protecting main's destination-prefixed path.
+// source-locally, consume accepted non-main declarations, and protect main's
+// destination-prefixed path. The zero value preserves existing behavior; see
+// the shared source-version contract in docs/config.md.
 type LayeredOptions[T any] struct {
 	Definition        Definition[T]
 	SourceVersionMode SourceVersionMode