The documentation already says the cache miss happens only at `ARG`
variable usage, not declaration, but there is a very common implicit
usage: `RUN`, which this commit documents even more, improving on #21790.
Also, use `definition` instead of `declaration`: it's the same thing, and
`definition` is already used in this documentation, contrary to
`declaration`.
Also, distinguish between "instructions" and "variables defined by `ARG`
instructions".
Signed-off-by: Thomas Riccardi <riccardi@systran.fr>
6ded7e8279