(travis) Forgotten bit while reverting f207111d6 (goes with 0d383895)
Correctly document the 'where' attribute

The documentation was correct when it was written 8 years ago in 4209f49a,
but then promptly broken 2 months later by 78060df8 (ironically one of the
corner stones of omnipotent chaining)

Fix documentation and add an extra test endorsing the current additive
behavior.