Clarify docs re fmt::make_format_args

[epruesse]

It might be worthwhile to make it very clear in the documentation that make_format_args will not make copies of anything, including named_arg arguments created on-the-fly with fmt::arg().

This looks good naively, will compile, and will actually (seem to) work correctly:

auto args = fmt::make_format_args(fmt::arg("key", value))  // BROKEN
auto msg = fmt::vformat("--{key}--", args);

It does, however, dereference parts of the stack that have been freed already...

Correct would be:

auto key_arg = fmt::arg("key", value);
auto args = fmt::make_format_args(key_arg);
auto msg = fmt::vformat("--{key}--", args);

[vitaut]

http://fmtlib.net/dev/api.html#_CPPv2N3fmt16make_format_argsEDpRK4Args already says that the returned object stores references

Constructs an format_arg_store object that contains references to arguments and can be implicitly converted to format_args.

but it might be worth clarifying further. PRs are welcome.

[mikelui]

Added documentation in #1051

[epruesse]

Thanks @mikelui