only diffs in edition notes

Author: mark-i-m

src/macros-by-example.md

@@ -51,17 +51,23 @@ designator is already known, and so only the name of a matched nonterminal comes
 after the dollar sign.
 
 In both the matcher and transcriber, the Kleene star-like operator indicates
-repetition. The Kleene star operator consists of `$` and parentheses, optionally
-followed by a separator token, followed by `*` or `+`. `*` means zero or more
-repetitions, `+` means at least one repetition. The parentheses are not matched or
-transcribed. On the matcher side, a name is bound to _all_ of the names it
-matches, in a structure that mimics the structure of the repetition encountered
-on a successful match. The job of the transcriber is to sort that structure
-out.
-
-> **Edition Differences**: In the 2018 edition, a new Kleene operator was
-> added: `?` means "at most once". This operator only works in edition 2018 and
-> onward. `?` does not accept a repetition separator, unlike `+` and `*`.
+repetition. The Kleene star operator consists of `$` and parentheses,
+optionally followed by a separator token, followed by `*`, `+`, or `?`. `*`
+means zero or more repetitions; `+` means _at least_ one repetition; `?` means
+at most one repetition. The parentheses are not matched or transcribed. On the
+matcher side, a name is bound to _all_ of the names it matches, in a structure
+that mimics the structure of the repetition encountered on a successful match.
+The job of the transcriber is to sort that structure out. Also, `?`, unlike `*`
+and `+`, does _not_ allow a separator, since one could never match against it
+anyway.
+
+> **Edition Differences**: The `?` Kleene operator did not exist before the
+> 2018 edition.
+
+> **Edition Differences**: Prior to the 2018 Edition, `?` was an allowed
+> separator token, rather than a Kleene operator. It is no longer allowed as a
+> separator as of the 2018 edition. This avoids ambiguity with the `?` Kleene
+> operator.
 
 The rules for transcription of these repetitions are called "Macro By Example".
 Essentially, one "layer" of repetition is discharged at a time, and all of them
@@ -93,8 +99,4 @@ Rust syntax is restricted in two ways:
    pairs when they occur at the beginning of, or immediately after, a `$(...)*`;
    requiring a distinctive token in front can solve the problem.
 
-> **Edition Differences**: As of the 2018 edition, `?` is not allowed as a
-> repetition separator. This is to remove ambiguity with the new `?` Kleene
-> operator.
-
 [RFC 550]: https://github.com/rust-lang/rfcs/blob/master/text/0550-macro-future-proofing.md