docs: clarify the purpose of the syntax parser directive

[dvdksn]

Addresses docker/docs#18946

[thernstig]

@dvdksn how would one see the built-in version used if not specifying the syntax directive? I have not seen anywhere it mentioned. Using docker info does not show it, but maybe it should?

[dvdksn]

@thernstig that would be nice. Unfortunately it's complicated to do, but hopefully one day. It's a long story, but the short version is that the docker commands can't get this information from BuildKit, because BuildKit itself isn't aware of what syntax version it ships with.

[thernstig]

@dvdksn if we just consider docs online then, where would this info be available which with BuildKit version -> docker frontend version?

[dvdksn]

@thernstig the best way to find out your default frontend version is to find the most recent stable Dockerfile release https://docs.docker.com/build/dockerfile/release-notes/ prior to the release of the buildkit version you're on.

[thernstig]

@thernstig the best way to find out your default frontend version is to find the most recent stable Dockerfile release https://docs.docker.com/build/dockerfile/release-notes/ prior to the release of the buildkit version you're on.

Thanks! Wish it was documented somehow more readily available for users to know this to get a releation between BuiltKit version <-> which dockerfile frontend version it contains in the docs (and later the CLI).