go/ast: document that ast.FilterFile always filters imports

Fixes #9248.

Change-Id: Id1c50af5eb35d7720b8f0a4d4881414baf061d56
Reviewed-on: https://go-review.googlesource.com/3241Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/ast/filter.go

@@ -221,8 +221,8 @@ func filterDecl(decl Decl, f Filter, export bool) bool {
 // names from top-level declarations (including struct field and
 // interface method names, but not from parameter lists) that don't
 // pass through the filter f. If the declaration is empty afterwards,
-// the declaration is removed from the AST. The File.Comments list
-// is not changed.
+// the declaration is removed from the AST. Import declarations are
+// always removed. The File.Comments list is not changed.
 //
 // FilterFile returns true if there are any top-level declarations
 // left after filtering; it returns false otherwise.