Corrected comments in mkdirs.cfg for P4MASTER_ID.
The P4MASTER_ID should only be set to the master server
(aka commit-server in a topology including edge servers).
In cases where the P4TARGET of a replica-of-an-edge is needed,
the P4TARGET value can be calculated dynamically. We leave
P4MASTER_ID to be statically configured, as typically it is
only configured once in the life of an instance. Other
aspects of the topology are dynamic and require dynamic
detection.