[css-will-change] Better wording for <animateable-feature>

[mrego]

The syntax for will-change property is the following one:

  Value: auto | <animateable-feature>#

However the definition of <animateable-feature> is quite broad, an excerpt:

it indicates that the author expects to animate or change the property

So a property like contain that is not animatable per spec is still considered part of these <animateable-feature> (see issue #4612).

I initially got confused about this, thinking that it shouldn't apply to contain because it was not animatable.
Could we find a better wording to avoid that kind of confusion? Or it was just me reading wrong the spec?