Update code comments about peer authenticaton

For historical reasons, the functions for peer authentication were
grouped under ident authentication.  But they are really completely
separate, so give them their own section headings.

src/backend/libpq/auth.c

@@ -73,6 +73,11 @@ static int	CheckSCRAMAuth(Port *port, char *shadow_pass, char **logdetail);
 
 static int	ident_inet(hbaPort *port);
 
+
+/*----------------------------------------------------------------
+ * Peer authentication
+ *----------------------------------------------------------------
+ */
 #ifdef HAVE_UNIX_SOCKETS
 static int	auth_peer(hbaPort *port);
 #endif
@@ -1967,6 +1972,12 @@ ident_inet_done:
 	return STATUS_ERROR;
 }
 
+
+/*----------------------------------------------------------------
+ * Peer authentication system
+ *----------------------------------------------------------------
+ */
+
 /*
  *	Ask kernel about the credentials of the connecting process,
  *	determine the symbolic name of the corresponding user, and check