Doc: fix remaining over-length entries in SQL keywords table.

Even after the tweaking I did in commit 5545b69ae, some of the
longer keywords mentioned in the SQL standard don't fit the
available space in PDF output.

I experimented with various solutions like putting such keywords
on their own table lines, but everything looked ugly or confusing
or both; worse, the weirdness also appeared in the HTML version,
which (normally) doesn't need it.

The best answer seems to be to insert &zwsp; into long keywords
so that they can be broken into two lines when, and only when,
needed.  It doesn't look too awful if the break happens after
an underscore --- and fortunately, all the problematic keywords
have underscores.

Discussion: https://postgr.es/m/6916.1589146280@sss.pgh.pa.us

doc/src/sgml/generate-keywords-table.pl

@@ -82,8 +82,13 @@ END
 
 foreach my $word (sort keys %keywords)
 {
+	# Insert zwsp's into very long keywords, so that they can be broken
+	# into multiple lines in PDF format (or narrow HTML windows).
+	my $printword = $word;
+	$printword =~ s/_/_&zwsp;/g if (length($printword) > 20);
+
 	print "   <row>\n";
-	print "    <entry><token>$word</token></entry>\n";
+	print "    <entry><token>$printword</token></entry>\n";
 
 	print "    <entry>";
 	if ($keywords{$word}{pg}{'unreserved'})