csprd01-17: Structure and Structural Elements: <sm> <em> justification

Hi Jörg,

> The annotation elements <sm> and <em> are just specialized cases of the 
> <mrk> element. Since they don't add any additional value to the inline 
> annotation markup, they could be subsumed under <mrk>. Therefore, a justification 
> of their existence would be most valuable.

The reason for <sm> and <em> elements to exist is the same as for <sc> and <ec> compared to <pc>: It's to allow overlapping
elements. For example when you have some metadata applied to a content across two sentences, and then the content is segmented or
marked up with some other overlapping metadata.

We went back and forth between this solution and duplicating <mrk>, and the pros/cons favored using <sm>/<em> at the end.

As for justifying those elements: there is a link in their definition to the Annotation section where, I think, it's explained there
in details. Do you think some additional text is needed in the <sm> and <em> section? Maybe another link to the section "Splitting
Annotations"?

Cheers,
-yves