Spellcheck: s/descendent/descendant/g

Spellcheck: s/descendent/descendant/g

I got a little annoyed by reading documentation paragraphs containing
both spellings within a few lines of each other.  My dictionary says
"descendant" is the preferred spelling, and it's certainly the majority
usage in our tree, so standardize on that.

For one usage in parallel.sgml, I thought it better to rewrite to avoid
the term altogether.