char: s/character/Unicode scalar value/

Tweak the definition of `char` to use the appropriate Unicode terminology.

Author: SimonSapin

src/doc/rust.md

@@ -3136,8 +3136,12 @@ machine.
 
 The types `char` and `str` hold textual data.
 
-A value of type `char` is a Unicode character,
-represented as a 32-bit unsigned word holding a UCS-4 codepoint.
+A value of type `char` is a [Unicode scalar value](
+http://www.unicode.org/glossary/#unicode_scalar_value)
+(ie. 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]` vector is effectively an UCS-4 / UTF-32 string.
 
 A value of type `str` is a Unicode string,
 represented as a vector of 8-bit unsigned bytes holding a sequence of UTF-8 codepoints.