[doc] Issue in "Aggregating a Package" The semantics of repository mapping are not documented

[hramrach]

Aggregating a Package:

https://openbuildservice.org/help/manuals/obs-user-guide/cha-obs-basicworkflow#sec-obs-basicworkflow-reuse-aggregating

The semantics of repository mapping are not documented, see openSUSE/open-build-service#18025 (comment) for details