Clarify/resolve contradictory sentences in Sec 4.1

[rsnikhil]

(Note: this was originally mentioned among various other points in this Issue: #138 (comment) but it was suggested that I should open a separate issue for this.)

In Section 4.1, we have:

The following information is mandatory:

    * The instruction_type of retired instructions for:
         * Jumps with a target that cannot be inferred from the source code;

and

The following information is optional:

    * The instruction_type of instructions for:
    ...
       * Jumps with a target that cannot be inferred from the source code;

These seem contradictory (both mandatory and optional)

[IainCRobertson]

@rsnikhil please review #162 - does this address your concerns?