Improve doc.

Author: ywxt

src/doc/unstable-book/src/compiler-flags/codegen-source-order.md

@@ -3,15 +3,10 @@
 ---
 
 This feature allows you to have a predictive and
-deterministic order for items after codegen.
+deterministic order for items after codegen, which
+is the same as in source code.
 
 For every `CodegenUnit`, local `MonoItem`s would
-be sorted by `(Span, DefPath, SymbolName)`, which
+be sorted by `(Span, SymbolName)`, which
 makes codegen tests rely on the order of items in
 source files work.
-
-Note: If you would like to use it, you have to
-manually sort items that are generated by a macro
-in lexicographical order in source files, because
-of the same spans owned by all items of a macro.
-So we have to distinguish them by `DefPath`s.