Fix docs for repo.gc() response format

SPEC/REPO.md

@@ -16,7 +16,11 @@ Where:
   - `quiet` writes a minimal output.
   - `stream-errors` stream errors.
 
-`callback` must follow `function (err, res) {}` signature, where `err` is an Error if the operation was not successful.
+`callback` must follow `function (err, res) {}` signature, where
+- `err` is an Error if the whole GC operation was not successful.
+- `res` is an array of objects that contains the following properties
+  - `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.
 
 If no `callback` is passed, a promise is returned.
 
@@ -80,3 +84,4 @@ ipfs.repo.version((err, version) => console.log(version))
 ```
 
 [1]: https://github.com/MikeMcl/bignumber.js/
+[cid]: https://www.npmjs.com/package/cids