branchcache-v3: use more explicit header line
The key-value approach is clearer and gives more rooms to have the format evolve
in a clear way. It also provides extension (like topic) simpler way to extend
the validation scheme.
This is just a small evolution, the V3 format is still a work in progress.
#require docutils gettext
$ $TESTDIR/check-gendoc sv
checking for parse errors