tweak str wording

Author: RalfJung

src/types/textual.md

@@ -8,13 +8,13 @@ or 0xE000 to 0x10FFFF range. It is immediate [Undefined Behavior] to create a
 `char` that falls outside this range. A `[char]` is effectively a UCS-4 / UTF-32
 string of length 1.
 
-A value of type `str` is a Unicode string, represented as an array of 8-bit
-unsigned bytes holding a sequence of UTF-8 encoded Unicode code points. Note
-that this is a library-level invariant: for the compiler and core language
-specification, `str` is the same as `[u8]`, but methods working on `str` may
-assume that the data in there is valid UTF-8 and may cause Undefined Behavior
-otherwise. Since `str` is a [dynamically sized type], it can only be
-instantiated through a pointer type, such as `&str`.
+A value of type `str` is represented the same way as `[u8]`, it is a slice of
+8-bit unsigned bytes. However, the Rust standard library makes extra assumptions
+about `str`: methods working on `str` assume and ensure that the data in there
+is valid UTF-8. Calling a `str` method with a non-UTF-8 buffer can cause
+[Undefined Behavior] now or in the future. \
+Since `str` is a [dynamically sized type], it can only be instantiated through a
+pointer type, such as `&str`.
 
 [Unicode scalar value]: http://www.unicode.org/glossary/#unicode_scalar_value
 [Undefined Behavior]: ../behavior-considered-undefined.md