doc/ref/spec.md: allow `...` in an ast.File

This is consistent with allowing ... in non-schema.

Other benefits:
- code generation can generate []Decl regardless of
whether it is used for a File or StructLit.
- once we have topological sort, `...` could be used
to hint insertion points when allowed at an arbitrary
position in the Decls list (although [string]: foo could
be used for that too).
- when adopting JSON Schema semantics, ...T may
has meaning at the file level as well.

Change-Id: Icfb299f951f5531119d33aeb5d89237eb64e33a1
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/5940
Reviewed-by: roger peppe <rogpeppe@gmail.com>
diff --git a/doc/ref/spec.md b/doc/ref/spec.md
index d7505b6..a0af53e 100644
--- a/doc/ref/spec.md
+++ b/doc/ref/spec.md
@@ -2859,8 +2859,9 @@
 it will be output instead of its enclosing file when exporting CUE
 to a data format
 
+<!-- TODO: allow ... anywhere in SourceFile and struct. -->
 ```
-SourceFile      = [ PackageClause "," ] { ImportDecl "," } { Declaration "," } .
+SourceFile = [ PackageClause "," ] { ImportDecl "," } { Declaration "," }  [ "..." ] .
 ```
 
 ```