Update documentation

Author: pdimov

doc/uuid/string_generator.adoc

@@ -14,8 +14,8 @@ struct string_generator
 {
     using result_type = uuid;
 
-    template<class Ch, class Traits, class Alloc>
-      constexpr uuid operator()( std::basic_string<Ch, Traits, Alloc> const& s ) const;
+    template<class Str>
+      constexpr uuid operator()( Str const& s ) const;
 
     template<class Ch>
       constexpr uuid operator()( Ch const* s ) const;
@@ -55,11 +55,14 @@ hhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh
 Invalid input will generate a `std::runtime_error` exception.
 
 ```
-template<class Ch, class Traits, class Alloc>
-  constexpr uuid operator()( std::basic_string<Ch, Traits, Alloc> const& s ) const;
+template<class Str>
+  constexpr uuid operator()( Str const& s ) const;
 ```
 
-Requires: :: The character type `Ch` of `s` must be one of `char`, `wchar_t`, `char8_t`, `char16_t`, `char32_t`.
+Requires: ::
+  `Str` must be a _string-like_ type; that is, one with a nested `value_type` type (`Ch`) and with a nested `traits_type` type.
+  `str.data()` must return `Ch const*`. `str.data() + str.size()` must be of type `Ch const*`.
+  `Ch` must be a character type (one of `char`, `wchar_t`, `char8_t`, `char16_t`, `char32_t`).
 
 Effects: :: Parses the string `s` into a `uuid` and returns the result.