[SE-0456] Clarifications (#2665)

* [SE-0456] Clarify UnsafeBufferPointer invariants

* [SE-0456] Clarify “Implications on adoption”

Author: glessard

proposals/0456-stdlib-span-properties.md

@@ -179,7 +179,7 @@ extension UnsafeMutableRawBufferPointer {
 }
 ```
 
-All of these unsafe conversions return a value whose lifetime is dependent on the _binding_ of the UnsafeBufferPointer. Note that this does not keep the underlying memory alive, as usual where the `UnsafePointer` family of types is involved. The programmer must ensure the following invariants for as long as the `Span` or `RawSpan` binding is valid:
+All of these unsafe conversions return a value whose lifetime is dependent on the _binding_ of the UnsafeBufferPointer. This dependency does not keep the underlying memory alive. As is usual where the `UnsafePointer` family of types is involved, the programmer must ensure the memory remains allocated while it is in use. Additionally, the following invariants must remain true for as long as the `Span` or `RawSpan` value exists:
 
   - The underlying memory remains initialized.
   - The underlying memory is not mutated.
@@ -220,7 +220,7 @@ This proposal is additive and ABI-compatible with existing code.
 
 ## Implications on adoption
 
-The additions described in this proposal require a new version of the Swift standard library and runtime.
+The additions described in this proposal require a version of the Swift standard library which include the `Span` and `RawSpan` types.
 
 ## Alternatives considered