Document how attribute relations & external IDs work (#57)

docs/aliases-and-external-ids.md

@@ -0,0 +1,36 @@
+# External IDs
+
+In your Catalog, entries have an ID, but they also have an external ID. The ID
+is a unique reference in our system, generated automatically for each entry,
+which cannot be set or changed. If you ever have something (say the
+catalog-importer) sync an entry into a type with an external ID, we’ll interpret
+that as the same entry as whatever already exists under that external ID.
+
+In practice, this means:
+* You can change the name and any attributes: we attach these to either the
+  external ID or the ID of the entry (whichever is available) and any renames
+  will transparently apply to custom field values and anywhere else the entry
+  was used.
+* If you happen to delete an entry but then try recreating it with the same
+  external ID, we’ll bring back the archived entry so you preserve any old
+  references such as incident custom field values.
+
+# Aliases
+
+The aliases are just about how you might reference a catalog entry via an
+attribute on some other entry.
+
+Say you have a team catalog type with a “Slack group” attribute which is of type
+“SlackUserGroup”. You could set that value of that attribute to either:
+* The ID of the Slack user group catalog entry (e.g. 01H4GFQPTNGNF0FFEGX2BT8811)
+* The external ID of the Slack user group catalog entry (e.g. S043VBNSBC3)
+* Or any of the aliases (e.g. my-user-group-handle)
+
+Then when you view that team entry in the dashboard or try navigating to its
+Slack group attribute, we’ll follow the above rules to find you the match.
+
+Aliases are *only* used for this matching, and nothing else, so:
+* They don't show up when you're searching for a catalog entry, either in the
+  catalog or to assign it to a custom field
+* They can't be edited via the Dashboard UI, only via the API, catalog-importer,
+  or Terraform.

docs/outputs.md

@@ -69,7 +69,9 @@ The `source` config for the output:
 - And similar that `external_id` should come from the `external_id` field.
 
 It's worth reading our guide on [expressions](expressions.md) to understand the
-syntax and what is possible in these fields.
+syntax and what is possible in these fields, and our guide on [external IDs and
+aliases](aliases-and-external-ids.md) to understand how these special fields
+work.
 
 The Backstage API Type (the catalog type we're creating for this output) will be
 given a single attribute: