libpq docs: be clearer about conninfo's 'hostaddr'

The previous wording was a bit too terse, too vague on the subject of
'host' and 'hostaddr' in connection specifications, which has caused
people to waste time trying to conform to rules because of
misunderstanding the whole thing; this small change should make things
clearer.

Author: Robert Haas, stemming from Fabien Coelho's complaints
Discussion: https://postgr.es/m/alpine.DEB.2.21.1808201323020.13832@lancre

doc/src/sgml/libpq.sgml

@@ -993,10 +993,14 @@ postgresql://%2Fvar%2Flib%2Fpostgresql/dbname
         your machine supports IPv6, you can also use those addresses.
         TCP/IP communication is
         always used when a nonempty string is specified for this parameter.
+        If this parameter is not specified, the value of <literal>host</literal>
+        will be looked up to find the corresponding IP address &mdash; or, if
+        <literal>host</literal> specifies an IP address, that value will be
+        used directly.
        </para>
 
        <para>
-        Using <literal>hostaddr</literal> instead of <literal>host</literal> allows the
+        Using <literal>hostaddr</literal> allows the
         application to avoid a host name look-up, which might be important
         in applications with time constraints. However, a host name is
         required for GSSAPI or SSPI authentication