Clarify String definition (#2044)

Changed string definitions in string.md and strings.md files according
to discussion #2028

src/std-types/string.md

@@ -4,7 +4,7 @@ minutes: 10
 
 # String
 
-[`String`][1] is the standard heap-allocated growable UTF-8 string buffer:
+[`String`][1] is a growable UTF-8 encoded string:
 
 ```rust,editable
 fn main() {