Improved mmap description.

applications/lofar2/designs/lofar2_unb2b_beamformer/lofar2_unb2b_beamformer.mmap.gold

 fpga_name = lofar2_unb2b_beamformer
 number_of_columns = 11
+# There can be multiple lines with a single key. The host should ignore unknown keys.
+# The lines with columns follow after the number_of_columns keys. The host should ignore
+# the extra columns in case the mmap contains more columns than the host expects.
+#
 # col 1: mm_port_name, if - then it is part of previous MM port.
 # col 2: number of peripherals, if - then it is part of previous peripheral.
 # col 3: number of mm_ports, if - then it is part of previous MM port.