Clarify that it is not like the shlib versions, and not like param.h's
__FreeBSD_version/osreldate either.
When config(8) was actively changing a while back, the interface between
config and the build system (eg: /sys/conf/files.* and Makefile.*) was
changing rapidly. configvers is a version number of that interface.
User specified config files do not have a version number. The decision
about whether a user supplied config file is syntactically valid or not
belongs to the parser and sanity checks, not an arbitary number.