Follow-up from MONGOID-4484

Based on this discussion: https://github.com/mongodb/mongoid/pull/5225#discussion_r851727093

we should clarify docs/reference/associations.txt

• Does accepts_nested_attributes_for coerce autosave: true on the association itself?
• What if you also explicitly set autosave: false?
• Or does the autosave only happen at runtime when passing in :album_attributes as a parameter to new / assign_attributes?