update UB list for safe target_feature

Author: RalfJung

src/attributes/codegen.md

@@ -75,7 +75,8 @@ The following is a list of the available feature names.
 
 #### `x86` or `x86_64`
 
-This platform requires that `#[target_feature]` is only applied to [`unsafe`
+Executing code with unsupported features is UB on this platform.
+Hence this platform requires that `#[target_feature]` is only applied to [`unsafe`
 functions][unsafe function].
 
 Feature     | Implicitly Enables | Description
@@ -133,7 +134,8 @@ Feature     | Implicitly Enables | Description
 
 #### `wasm32` or `wasm64`
 
-This platform allows `#[target_feature]` to be applied to both safe and
+Executing code with unsupported features is allowed (i.e., is not UB) on this platform.
+Hence this platform allows `#[target_feature]` to be applied to both safe and
 [`unsafe` functions][unsafe function].
 
 Feature     | Description

src/behavior-considered-undefined.md

@@ -33,7 +33,7 @@ code.
   is immutable, unless that data is contained within an [`UnsafeCell<U>`].
 * Invoking undefined behavior via compiler intrinsics.
 * Executing code compiled with platform features that the current platform
-  does not support (see [`target_feature`]).
+  does not support (see [`target_feature`]), *except* if the platform explicitly documents this to be safe.
 * Calling a function with the wrong call ABI or unwinding from a function with the wrong unwind ABI.
 * Producing an invalid value, even in private fields and locals. "Producing" a
   value happens any time a value is assigned to or read from a place, passed to