Clarify lifetimes of named_arg parameters

doc/api.rst

@@ -54,7 +54,7 @@ participate in an overload resolution if the latter is not a string.
 Named arguments
 ---------------
 
-.. doxygenfunction:: fmt::arg(string_view, const T&)
+.. doxygenfunction:: fmt::arg(const S&, const T&)
 
 Named arguments are not supported in compile-time checks at the moment.
 

include/fmt/core.h

@@ -1200,6 +1200,7 @@ const unsigned long long format_arg_store<Context, Args...>::TYPES =
   Constructs an `~fmt::format_arg_store` object that contains references to
   arguments and can be implicitly converted to `~fmt::format_args`. `Context`
   can be omitted in which case it defaults to `~fmt::context`.
+  See `~fmt::arg` for lifetime considerations.
   \endrst
  */
 template <typename Context = format_context, typename... Args>
@@ -1384,6 +1385,12 @@ typename buffer_context<Char>::type::iterator vformat_to(
   \rst
   Returns a named argument to be used in a formatting function.
 
+  The named argument holds a reference and does not extend the lifetime
+  of its arguments.
+  Consequently, a dangling reference can accidentally be created.
+  The user should take care to only pass this function temporaries when
+  the named argument is itself a temporary, as per the following example.
+
   **Example**::
 
     fmt::print("Elapsed time: {s:.2f} seconds", fmt::arg("s", 1.23));