Improve Document API (#348)

* Improve Document API

* Improve Document API

* Review

---------

Co-authored-by: Simran Spiller <[email protected]>

Author: jwierzbo

site/content/3.10/concepts/data-structure/documents/_index.md

@@ -60,15 +60,17 @@ Example document:
 All documents contain special attributes at the top-level that start with an 
 underscore, known as **system attributes**:
 
-- The **document identifier** is stored as a string in the `_id` attribute.
 - The **document key** is stored as a string in the `_key` attribute.
+- The **document identifier** is stored as a string in the `_id` attribute.
 - The **document revision** is stored as a string in the `_rev` attribute.
 
 You can specify a value for the `_key` attribute when creating a document.
+The `_id` attribute is automatically set based on the collection and `_key`.
 The `_id` and `_key` values are immutable once the document has been created.
 The `_rev` value is maintained by ArangoDB automatically.
 
 Edge documents in edge collections have two additional system attributes:
+
 - The document identifier of the source vertex stored in the `_from` attribute.
 - The document identifier of the target vertex stored in the `_to` attribute.
 
@@ -80,6 +82,9 @@ you should avoid using own attribute names starting with an underscore.
 Each document has a unique **document key** (or _primary key_) which identifies
 it within its collection.
 
+To distinguish between documents from multiple collections, see
+[Document identifiers](#document-identifiers).
+
 A document key uniquely identifies a document in the collection it is
 stored in. It can and should be used by clients when specific documents
 are queried. The document key is stored in the `_key` attribute of
@@ -157,6 +162,10 @@ forward slash (`/`), like `collection-name/document-key`.
 See [Collection names](../collections.md#collection-names) and
 [Document keys](#document-keys) for information about the allowed characters.
 
+When working with documents from multiple collections, you can see what
+collections they are from by looking at the `_id` attribute values and tell
+documents from different collections but the same keys apart.
+
 ### Document revisions
 
 Every document in ArangoDB has a revision, stored in the system attribute