document weird behavior

[bjaglin]

Follows #378 & scalacenter/scalafix#1874

The sbt behavior or completions for aggregating tasks (typically scalafix on the root project) is interesting as all aggregated projects contribute to the possible completion values.

• everything goes well if at one (resp. several) aggregated project(s) can validate all the arguments, in which case that one runs (resp. these ones run) and the others are silently skipped
• if provided several arguments valid each of them in one different aggregated project, the invocation will fail, showing the validation error on the aggregating project.