Multi-asset documentation (#1112)

* feat: adding provenance

* chore: lint

* feat: adding multi-asset stage and typo

* chore: examples

* refactor: adding validator and renaming

tests are still todo

* tests: adding tests, minor fixes

* tests: examples and line length

* fix: multi_session->multi_asset

* fix: session suffix to asset

* fix: typo

* fix: more typos

* docs: adding docs for multi-asset

* docs: remove confusing statement, add storage q

docs/source/quality_control.md

@@ -102,6 +102,14 @@ Each metric is associated with a reference figure, image, or video. The QC porta
 
 By default the QC portal displays dictionaries as tables where the values can be edited. We also support a few special cases to allow a bit more flexibility or to constrain the actions that manual annotators can take. Install the `aind-qcportal-schema` package and set the `value` field to the corresponding pydantic object to use these. 
 
-### Multi-session QC
+### Multi-asset QC
 
-[Details coming soon, this is under discussion]
+During analysis there are many situations where multiple data assets need to be pulled together, often for comparison. For example, FOVs across imaging sessions or recording sessions from a chronic probe might need to get matched up across days. When a `QCEvaluation` is being calculated from multiple assets it should be tagged with `Stage:MULTI_ASSET` and each of its `QCMetric` objects needs to track the assets that were used to generate that metric in the `evaluated_assets` list.
+
+**Q: Where do I store multi-asset QC?**
+
+You should follow the preferred/alternate workflows described above. If your multi-asset analysis pipeline generates a new data asset, put the QC there. If your pipeline does not generate an asset, push a copy of each `QCEvaluation` back to **each** individual data asset.
+
+**Q: I want to be able to store data about each of the evaluated assets in this metric**
+
+Take a look at the `MultiAssetMetric` class in `aind-qc-portal-schema`. It allows you to pass a list of values which will be matched up with the `evaluated_assets` names. You can also include options which will appear as dropdowns or checkboxes.