RelatedTo "and" and arrays
I don’t know what it is – I swear: every time – but the next thing I tried, immediately after posting the question was this:

{% set related = ['and',] %}
{% for relation in relations %}
    {% set related = related|merge([{targetElement:relation}]) %}
{% endfor %}

{% paginate craft.entries
    .section('documents')
    .relatedTo(related)
    as entries %}   


That works.