[FEATURE] Document how explicit exception report maps determine how they match a key to an exception

[gdurandrexel]

For explicit exception report mode/handler maps, to determine which handler to call, the implementation calls toString on the captured exception and searches for a map key inside this string. This might not work if the exception overrides toString.

This behaviour should be documented: the map key must be a part of the string returned by exception.toString() (case insensitive).