Clarify lifetimes of named_arg parameters (#1051)

* Clarify usage of fmt::arg

Document that fmt::arg takes a non-owning
reference, even if that reference is to
a temporary. As such, users should make sure
the lifetime of the reference lasts as long
as the named argument.

* Clean up language

Remove mentions of `std::reference_wrapper` and rvalues
in favor of more common terminology like dangling references.

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));