Clarify the guarantees of Vec::as_ptr and Vec::as_mut_ptr when there's no allocation

Author: SkiFire13

alloc/src/vec/mod.rs

@@ -1107,7 +1107,8 @@ impl<T, A: Allocator> Vec<T, A> {
         self
     }
 
-    /// Returns a raw pointer to the vector's buffer.
+    /// Returns a raw pointer to the vector's buffer, or a dangling raw pointer
+    /// valid for zero sized reads if the vector didn't allocate.
     ///
     /// The caller must ensure that the vector outlives the pointer this
     /// function returns, or else it will end up pointing to garbage.
@@ -1144,7 +1145,8 @@ impl<T, A: Allocator> Vec<T, A> {
         ptr
     }
 
-    /// Returns an unsafe mutable pointer to the vector's buffer.
+    /// Returns an unsafe mutable pointer to the vector's buffer, or a dangling
+    /// raw pointer valid for zero sized reads if the vector didn't allocate.
     ///
     /// The caller must ensure that the vector outlives the pointer this
     /// function returns, or else it will end up pointing to garbage.