Clarify the stackBefore() documentation

The concept of "stack before" may be clear to English native speakers, but it is not for many others.

Words like "before" or "back" are controversial in English even for non native speakers when used with phrasal verbs (see this post).

For many other languages, those words are normally more consistent: for instance, considering stacking and visual order, "before" may be intended as "in front of" or "ahead of".

The documentation should clarify this aspect in a more "international fashion", clarifying that "stack item x before item y" actually means "put item x below item y".