docs: update repo.gc API doc

Author: Alan Shaw

SPEC/REPO.md

@@ -20,18 +20,19 @@ Where:
 
 | Type | Description |
 | -------- | -------- |
-| `Promise<Array>` | An array of objects |
+| `AsyncIterable<Object>` | An async iterable that yields objects describing nodes that were garbage collected |
 
-each object contains the following properties:
+Each yielded object contains the following properties:
 
-- `err` is an Error if it was not possible to GC a particular block.
+- `err` is an `Error` if it was not possible to GC a particular block.
 - `cid` is the [CID][cid] of the block that was Garbage Collected.
 
 **Example:**
 
 ```JavaScript
-const res = await ipfs.repo.gc()
-console.log(res)
+for await (const res of ipfs.repo.gc()) {
+  console.log(res)
+}
 ```
 
 #### `repo.stat`