update language in spec about merge + replace

[miabbott]

Perhaps it is just me not reading the spec carefully enough, but I made the mistake of trying to merge two FCCT yaml docs like this:

variant: fcos
version: 1.1.0
ignition:
  config:
    merge:
      - local: config1.yaml
      - local: config2.yaml

Which resulted in an Ignition JSON document that was not parsed correctly by Ignition. While this was the correct behavior, I think it is easy to make the mistake of seeing merge in the spec and trying something like I did.

Would it be possible to update the language in the spec for the ignition portion to be clear that merge and replace expect Ignition JSON documents and not FCCT YAML docs?

[bgilbert]

See also #118.