Fix summary of rust-2021/reserving-syntax.

Author: m-ou-se

src/rust-2021/reserving-syntax.md

@@ -3,7 +3,8 @@
 ## Summary
 
 - `any_prefix#..`, `any_prefix".."`, and `any_prefix'..'` are reserved syntax, and no longer tokenize.
-- These did not parse as valid Rust in any edition, except in arguments to macros.
+- This is mostly relevant to macros. E.g. `quote!{ #a#b }` is no longer accepted.
+- It doesn't treat keywords specially, so e.g. `match".." {}` is no longer accepted.
 - Insert whitespace to avoid errors.
 
 ## Details