Describe RAKUDO_EXCEPTIONS_HANDLER

specifically the expected properties of the handler class

Author: zoffixznet

doc/Programs/00-running.pod6

@@ -92,7 +92,11 @@ Causes the module loader to print debugging information to standard error.
 
 =defn C<RAKUDO_EXCEPTIONS_HANDLER> (I<Str>; C<src/core/Exception.pm>)
 Specifies the name of a custom exceptions handler. For example, to load and use
-C<Exceptions::JSON> class, set this env var to C<JSON>
+C<Exceptions::JSON> class that comes with Rakudo, set this env var to C<JSON>.
+The exception handling class needs to implement method C<process> that takes
+a single positional argument: the exception object. If that method returns
+a truthy value, the exception will also be printed to the terminal, as if
+the exception handler was not specified.
 
 =defn C<RAKUDO_NO_DEPRECATIONS> (I<Bool>; C<src/core/Deprecations.pm>)
 If true, suppresses deprecation warnings triggered by the C<is DEPRECATED> trait.