clarify wording (#1931)

README.md

@@ -444,8 +444,9 @@ to set `true`; it's effectively a shortcut for `foo=true`).
   being compressed into `1/0`, which may cause performance issues on Chrome.
 
 - `side_effects` -- default `false`. Pass `true` to potentially drop functions
-marked as "pure".  (A function is marked as "pure" via the comment annotation
-`/* @__PURE__ */` or `/* #__PURE__ */`)
+marked as "pure".  A function call is marked as "pure" if a comment annotation
+`/*@__PURE__*/` or `/*#__PURE__*/` immediately precedes the call. For example:
+`/*@__PURE__*/foo()`;
 
 
 ### The `unsafe` option