Move the @unsafe for unsafe conformances to where @unchecked is for Sendable

Thank you for the suggestion, Xiaodi!

Author: DougGregor

proposals/nnnn-strict-memory-safety.md

@@ -282,15 +282,16 @@ extension ConformsToP: P {
 }
 ```
 
-To suppress this warning, one can place `@unsafe` on the extension (or type) where the conformance to `P` is supplied. This notes that the conformance itself is unsafe:
+To suppress this warning, one can place `@unsafe` on the conformance to `P` is supplied. This notes that the conformance itself is unsafe:
 
 ```swift
-@unsafe
-extension ConformsToP: P {
+extension ConformsToP: @unsafe P {
   @unsafe func f() { }
 }
 ```
 
+Use of an `@unsafe` conformance for any reason (e.g., when that conformance is needed to call a generic function with a `ConformsToP` requirement) is diagnosed as an unsafe use, much like use of an `@unsafe` declaration.
+
 ### Strict safety mode and escalatable warnings
 
 The strict memory safety mode can be enabled with the new compiler flag `-strict-memory-safety`.