Document the corpus filter on RemoveSelection

https://redmine.teklia.com/issues/7882

The @extend_schema_view that documents RemoveSelection includes the optional id filter, which is used to remove elements from the selection one at a time, but it does not include the corpus filter, which unselects all elements of a corpus at once. This lead us to believe we did not have this filter yet, when we always had it, and makes this filter unusable in the API client.

• A help_text should be added to the existing id field to document that when set, only this element ID will be removed from the selection.
• A new corpus field should be added with another help_text to document that when set, only elements with this corpus ID will be removed from the selection.

The behavior of the backend does not change, this only updates the docs found in /api-docs/ to reflect the existing behavior.