Clarify lifetimes of named_arg parameters (#1051)
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.
cppformat/cppformat,cppformat/cppformat,cppformat/cppformat,alabuzhev/fmt,alabuzhev/fmt,alabuzhev/fmt