Document various meta.yaml components available in the spec section

[amisevsk]

PR eclipse-che/che-plugin-registry#150 adds basic meta.yaml documentation, but it lacks detailed docs for the new spec section, which comes from che-plugin.yaml. If there are docs available for the various fields in the former che-plugin.yaml, they should be merged with the docs for meta.yamls.

E.g.:

• What is the purpose of the various attributes available on ports? The only docs I've found are for Che 6 servers (and that is mentioned in the PR); I can figure out a few from looking at the code but others are entirely opaque -- what to type and path attributes do for a Che 7 workspace?
• Do volumes in .spec.containers.image.volumes have to be created separately?
• Are the current attributes reflective of what Che 7 is currently or holdovers from legacy code (e.g. cookiesAuthEnabled vs secure vs discoverable vs public-- can this be simplified?)

More questions can be found in @tsmaeder's review of PR #150

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