docs: update since buildinfo removal

[crazy-max]

follow-up moby/buildkit#3582

Update docs since buildinfo has been removed from BuildKit. We need to keep some buildinfo logic in buildx for backward compat with old builders. Such as:

buildx/build/build.go

Lines 559 to 562 in 4a73abf

	// inline buildinfo attrs from build arg
	if v, ok := opt.BuildArgs["BUILDKIT_INLINE_BUILDINFO_ATTRS"]; ok {
	e.Attrs["buildinfo-attrs"] = v
	}

Keeping in draft until BuildKit 0.12.

[jedevc]

🎉