work on char/str descriptions

Author: RalfJung

src/types/textual.md

@@ -2,15 +2,20 @@
 
 The types `char` and `str` hold textual data.
 
-A value of type `char` is a [Unicode scalar value] (i.e. a code point that
-is not a surrogate), represented as a 32-bit unsigned word in the 0x0000 to
-0xD7FF or 0xE000 to 0x10FFFF range. A `[char]` is effectively a UCS-4 / UTF-32
-string.
+A value of type `char` is a [Unicode scalar value] (i.e. a code point that is
+not a surrogate), represented as a 32-bit unsigned word in the 0x0000 to 0xD7FF
+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 code points. Since `str` is a
-[dynamically sized type], it is not a _first-class_ type, but can only be
-instantiated through a pointer type, such as `&str`.
+unsigned bytes holding a sequence of UTF-8 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`.
 
 [Unicode scalar value]: http://www.unicode.org/glossary/#unicode_scalar_value
+[Undefined Behavior]: ../behavior-considered-undefined.html
 [dynamically sized type]: ../dynamically-sized-types.md