Update the prose for <witness>

[npcole]

In reviewing #1868 and merging the fix, we realized that the prose for <witness> needs updating. It is currently manuscript-focused, and does not really reflect the element now.

[martinascholger]

VF2F: @npcole can you remind us what the exact concern was? Perhaps we should add an example illustrating the use of witness with msDesc/bibl/object children instead of just text description or mixed, unstructured content.

[sydb]

We are not entirely sure what part of the prose about <witness> is being objected to. Probably 12.1.4? Anyway, Council thinks it is useful to add a non-mss example. @ebeshero (or @raffazizzi) will supply an example (here on this ticket) for assignees to work into the Guidelines.