E.15 Clarify when a rethrow would slice to `std::exception`

[N-Dekker]

Avoided a potential misunderstanding that throw e would slice any e to std::exception.

The note about rethrowing appears to be written when the example of this item still did catch (const exception& e), which was prior to commit d568d190f65a2cd9d3a3fde74cddbc5b5489bd99, "Generalized E.15, closes #1848".