[string.syn] Index basic_string typedefs. (#4125)

Author: Eelis

source/strings.tex

@@ -658,11 +658,11 @@
       erase_if(basic_string<charT, traits, Allocator>& c, Predicate pred);
 
   // \tcode{basic_string} typedef names
-  using string    = basic_string<char>;
-  using u8string  = basic_string<char8_t>;
-  using u16string = basic_string<char16_t>;
-  using u32string = basic_string<char32_t>;
-  using wstring   = basic_string<wchar_t>;
+  using @\libglobal{string}@    = basic_string<char>;
+  using @\libglobal{u8string}@  = basic_string<char8_t>;
+  using @\libglobal{u16string}@ = basic_string<char16_t>;
+  using @\libglobal{u32string}@ = basic_string<char32_t>;
+  using @\libglobal{wstring}@   = basic_string<wchar_t>;
 
   // \ref{string.conversions}, numeric conversions
   int stoi(const string& str, size_t* idx = nullptr, int base = 10);