add a bit to syntax intro

Author: mark-i-m

src/syntax-intro.md

@@ -6,3 +6,8 @@ out that doing even this involves a lot of work, including lexing, parsing,
 macro expansion, name resolution, conditional compilation, feature-gate
 checking, and validation of the AST. In this chapter, we take a look at all
 of these steps.
+
+Notably, there isn't always a clean ordering between these tasks. For example,
+macro expansion relies on name resolution to resolve the names of macros and
+imports. And parsing requires macro expansion, which in turn may require
+parsing the output of the macro.