Exception formatting is undocumented

[robinchrist]

Formatting for exceptions exists (e.g. via https://github.com/fmtlib/fmt/pull/3062 or https://github.com/fmtlib/fmt/pull/3076). This is not documented (missing in the list at https://github.com/fmtlib/fmt/blob/cc2ba8f9ede4e5ae3262f43f3e4d07a22a9acdfc/doc/api.md#L491-L505).

Additionally, there seem to be different ways to format exceptions (without any specified and with {:t}), as can be seen from the tests: https://github.com/fmtlib/fmt/blob/cc2ba8f9ede4e5ae3262f43f3e4d07a22a9acdfc/test/std-test.cc#L311-L318

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