Doc: describe the "options" allowed in an ECPG connection target string.

These have been there a long time, but their format was never explained
in the docs.  Per complaint from Yusuke Egashira.

Discussion: https://postgr.es/m/848B1649C8A6274AA527C4472CA11EDD5FC70CBE@G01JPEXMBYT02

doc/src/sgml/ecpg.sgml

@@ -177,6 +177,19 @@ EXEC SQL CONNECT TO <replaceable>target</replaceable> <optional>AS <replaceable>
    SQL string literal, or a reference to a character variable.
   </para>
 
+  <para>
+   If the connection target includes any <replaceable>options</replaceable>,
+   those consist of
+   <literal><replaceable>keyword</replaceable>=<replaceable>value</replaceable></literal>
+   specifications separated by ampersands (<literal>&amp;</literal>).
+   The allowed key words are the same ones recognized
+   by <application>libpq</application> (see
+   <xref linkend="libpq-paramkeywords"/>).  Spaces are ignored before
+   any <replaceable>keyword</replaceable> or <replaceable>value</replaceable>,
+   though not within or after one.  Note that there is no way to
+   write <literal>&amp;</literal> within a <replaceable>value</replaceable>.
+  </para>
+
   <para>
    The <replaceable>connection-name</replaceable> is used to handle
    multiple connections in one program.  It can be omitted if a