restructured section 4.1.10 Voided

As in my previous merges, I restructured the section for added clarity.
Please check for completeness and correctness. Note that I changed
'should' into 'must' in the original sentence that read: 'should mark
the target statement as voided using the "voided" field.' (line 1226)

I.m.h.o a core functionality as this is not optional. By all means,
change back to SHOULD if MUST is too stringent; I did not follow all
discussion around this.
Rewrite of short sections 4.1.1, 4.1.7 and 4.1.8

ID, Timestamp and Stored. Just some restructuring of requirements and
descriptions. Hope there are no whitespace issues this time.
Merge pull request #58 from paul-esch/master

couple of small rewrites in 4.1.1, 4.1.7 and 4.1.8