chore: improve exception handling notes around lasti

This change improves the description of the new exception table,
focusing on describing the meaning of the lasti flag, and when one
should expect it to be set.

Author: P403n1x87

Objects/exception_handling_notes.txt

@@ -92,6 +92,16 @@ If there is an entry, then:
  3. push the exception to the stack.
  4. jump to the target offset and resume execution.
 
+Currently, the only opcode that requires the lasti flag to be set is RERAISE.
+That is, if the jump described by an exception table points to an offset where
+a RERAISE (with a non-zero oparg) occurs, the lasti is expected to be on the
+stack, and therefore the lasti flag of the exception table entry must be set.
+The last example gives an illustration of this. The first table entry describes
+a jump where no RERAISE is executed. The second entry describes a jump where a
+RERAISE with oparg 1 is executed. This means that the RERAISE will consume the
+TOS and assume it is the lasti that was pushed when the exception occurred, as
+instructed by the lasti flag.
+
 
 Format of the exception table
 -----------------------------