Exception formatting is undocumented

[robinchrist]

Formatting for exceptions exists (e.g. via #3062 or #3076).
This is not documented (missing in the list at

fmt/doc/api.md

Lines 491 to 505 in cc2ba8f

	<a id="std-api"></a>
	## Standard Library Types Formatting
	`fmt/std.h` provides formatters for:
	- [`std::atomic`](https://en.cppreference.com/w/cpp/atomic/atomic)
	- [`std::atomic_flag`](https://en.cppreference.com/w/cpp/atomic/atomic_flag)
	- [`std::bitset`](https://en.cppreference.com/w/cpp/utility/bitset)
	- [`std::error_code`](https://en.cppreference.com/w/cpp/error/error_code)
	- [`std::filesystem::path`](https://en.cppreference.com/w/cpp/filesystem/path)
	- [`std::monostate`](https://en.cppreference.com/w/cpp/utility/variant/monostate)
	- [`std::optional`](https://en.cppreference.com/w/cpp/utility/optional)
	- [`std::source_location`](https://en.cppreference.com/w/cpp/utility/source_location)
	- [`std::thread::id`](https://en.cppreference.com/w/cpp/thread/thread/id)
	- [`std::variant`](https://en.cppreference.com/w/cpp/utility/variant/variant)

).

Additionally, there seem to be different ways to format exceptions (without any specified and with {:t}), as can be seen from the tests:

fmt/test/std-test.cc

Lines 311 to 318 in cc2ba8f

	try {
	using namespace my_ns1::my_ns2;
	throw my_exception("My Exception");
	} catch (const std::exception& ex) {
	EXPECT_EQ("my_ns1::my_ns2::my_exception: My Exception",
	fmt::format("{:t}", ex));
	EXPECT_EQ("My Exception", fmt::format("{:}", ex));
	}

This is undocumented in https://github.com/fmtlib/fmt/blob/cc2ba8f9ede4e5ae3262f43f3e4d07a22a9acdfc/doc/syntax.md. The only reference in the docs to t is A horizontal-tab character. for chrono

[vitaut]

Formatting of std::exception is still WIP/experimental and exact format string syntax is subject to change but I added it to the list in https://fmt.dev/dev/api/#standard-library-types-formatting.