Update documentation for importer

[linakrisztian]

The API docs for the importer should be updated:

• currently API docs for importer extent are wrong: not a string (STAC)
• Info at the top: importer will always be executed as first step within a PC
  • thus no g.region command before importer possible + also no g.region within previous PC possible, since it is not written back within mapset merge ---> for this also see Improve region settings for processing actinia-core#493
• general update of documentation