[Doc] Improve hostssl related descriptions and option presentation

Motivated by a recent complaint [1] I found the hostssl related material in our docs quite verbose and even repetitive.  Some of that is normal since we have both an overview/walk-through section as well as a reference section.  But the overview in particular was self-repetitive.  Here is a first pass at some improvements.  The commit message contains both the intent of the patch and some thoughts/questions still being considered.

David J.

[1] https://www.postgresql.org/message-id/170672433664.663.11895343120533141715%40wrigleys.postgresql.org