.github: clarify what we mean by "latest release"

In Go, "latest" tends to mean "latest stable" due to `@latest` being a
valid version suffix when referring to Go modules.

However, in the broader software ecosystem, it's less clear.
For example, "latest" tags in Docker tend to mean any latest version,
which might be stable or not.
Even if a user only considers our releases to be valid versions,
we also do pre-releases like betas.

Clarify what we mean.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: I265ad5dac075f4f4dc214fd974ef9a6dcefae7f1
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/548989
Reviewed-by: Paul Jolly <paul@myitcv.io>
Unity-Result: CUEcueckoo <cueckoo@cuelang.org>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>