Documentation about the contributor role does not reflect AtoM's default permissions

The documentation currently lists contributors being able to "search, browse, create, edit/update, view draft and export descriptions (published archival descriptions, authority records, archival institutions)". This is not accurate — they cannot edit authority records or archival institutions, some of those permissions are reserved for editors and admins.

Locate the documentation section describing the contributor role and compare its permissions list with AtoM's default permissions. Correct the entries for authority records and archival institutions, then review the surrounding role descriptions to ensure the documented contributor capabilities are accurate.

Written by the indexing model from the issue text.