Update schema proposals editor abilities (#939)

* Update schema editor abilities

* Copyedit

* Vale

src/content/graphos/delivery/schema-proposals/creation.mdx

@@ -86,8 +86,18 @@ You can directly edit one or more subgraph schemas from a proposal's **Editor**
   alt="Schema proposals editor in GraphOS Studio"
 />
 
-To start a new subgraph, click **+Add new** from the subgraph schema selector in the left navigation.
-You can also delete existing subgraphs and rename newly created ones by clicking the **•••** menu to the right of a subgraph's name.
+The proposals editor provides features like autocomplete, hovering over types and directives for more information, and syntax and schema validation.
+
+The editor also lets you run composition on your proposed changes to uncover composition errors before you finalize your proposal.
+To run composition, click the icon with three boxes in the top right of the editor.
+Next to the button, you can toggle on **Compose on change** so that composition automatically runs on every change.
+
+<Note>
+
+For extremely large, complex schemas, composing on each change may affect browser performance.
+If you notice your browser lagging, consider keeping **Compose on change** off.
+
+</Note>
 
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