Doc: further clarify libpq's description of connection string URIs.

Doc: further clarify libpq's description of connection string URIs.

Break the synopsis into named parts to make it less confusing.
Make more than zero effort at applying SGML markup.  Do a bit
of copy-editing of nearby text.

The synopsis revision is by Alvaro Herrera and Paul Förster,
the rest is my fault.  Back-patch to v10 where multi-host
connection strings appeared.

Discussion: https://postgr.es/m/6E752D6B-487C-463E-B6E2-C32E7FB007EA@gmail.com