Doc: minor improvements to SNI documentation.

Doc: minor improvements to SNI documentation.

My attention was drawn to this new documentation by overlength-line
complaints in the PDF docs builds: the synopsis for hostname lines was
too wide.  I initially thought of shortening the parameter names to
fit, but it turns out that adding <optional> markup is enough to
persuade DocBook to break the line, and that seems more helpful
anyway.

While here, I couldn't resist some copy-editing, mostly being
consistent about whether to use Oxford commas or not.  The biggest
change was to re-order the entries in the hostname-values table to
match the running text.