Restructure log_line_prefix options to be clearer:

	#log_line_prefix = ''   # %u = user name
                                # %d = database name
                                # %r = remote host and port
                                # %h = remote host
                                # %p = PID
                                # %t = timestamp
                                # %m = timestamp with milliseconds
                                # %i = command tag
                                # %c = session id
                                # %l = session line number
                                # %s = session start timestamp
                                # %x = transaction id
                                # %q = stop here in non-session processes
                                # %% = '%'
                                # e.g. '<%u%%%d> '