Update edition differences for unsafe extern blocks

This follows the style that the main text should describe the current
edition, and that edition differences are in a blockquote, and they
describe what is different.

Author: ehuss

src/items/external-blocks.md

@@ -33,10 +33,11 @@ Calling functions or accessing statics that are declared in external blocks is o
 r[items.extern.namespace]
 The external block defines its functions and statics in the [value namespace] of the module or block where it is located.
 
-**Edition differences**: Starting in the 2024 edition, the `unsafe` keyword is
-required to appear before the `extern` keyword on external blocks. In previous
-editions, it is accepted but not required. The `safe` and `unsafe` item qualifiers
-are only allowed if the external block itself is marked as `unsafe`.
+r[items.extern.unsafe-required]
+The `unsafe` keyword is semantically required to appear before the `extern` keyword on external blocks.
+
+r[items.extern.edition2024]
+> **Edition differences**: Prior to the 2024 edition, the `unsafe` keyword is optional. The `safe` and `unsafe` item qualifiers are only allowed if the external block itself is marked as `unsafe`.
 
 ## Functions
 

src/unsafe-keyword.md

@@ -81,7 +81,8 @@ r[unsafe.extern]
 
 The programmer who declares an [external block] must assure that the signatures of the items contained within are correct. Failing to do so may lead to undefined behavior.  That this obligation has been met is indicated by writing `unsafe extern`.
 
-**Edition differences**: Prior to edition 2024, `extern` blocks were allowed without being qualified as `unsafe`.
+r[unsafe.extern.edition2024]
+> **Edition differences**: Prior to edition 2024, `extern` blocks were allowed without being qualified as `unsafe`.
 
 [external block]: items/external-blocks.md