Graphdriver docs (#22003)

* Fix closing strings in graphdriver plugin documentation

Signed-off-by: Thomas Gazagnaire <thomas@gazagnaire.org>

* Fix documenation for Err type in graphdriver plugins

Fix https://github.com/docker/go-plugins-helpers/issues/24

Signed-off-by: Thomas Gazagnaire <thomas@gazagnaire.org>

* Add missing MountLabel argument in graphdriver plugin documentation

The real `Create` seems also to take more arguments (the `storageOpt`) which
are not exposed to the plugin API (yet?).

Signed-off-by: Thomas Gazagnaire <thomas@gazagnaire.org>

* Add missing CreateReadWrite in graphdriver plugin documentation

Signed-off-by: Thomas Gazagnaire <thomas@gazagnaire.org>

experimental/plugins_graphdriver.md

@@ -37,11 +37,11 @@ through from the user.
 **Response**:
 ```
 {
-  "Err": null
+  "Err": ""
 }
 ```
 
-Respond with a string error if an error occurred.
+Respond with a non-empty string error if an error occurred.
 
 
 ### /GraphDriver.Create
@@ -51,22 +51,35 @@ Respond with a string error if an error occurred.
 {
   "ID": "46fe8644f2572fd1e505364f7581e0c9dbc7f14640bd1fb6ce97714fb6fc5187",
   "Parent": "2cd9c322cb78a55e8212aa3ea8425a4180236d7106938ec921d0935a4b8ca142"
+  "MountLabel": ""
 }
 ```
 
-Create a new, empty, filesystem layer with the specified `ID` and `Parent`.
-`Parent` may be an empty string, which would indicate that there is no parent
-layer.
+Create a new, empty, read-only filesystem layer with the specified
+`ID`, `Parent` and `MountLabel`. `Parent` may be an empty string,
+which would indicate that there is no parent layer.
 
 **Response**:
 ```
 {
-  "Err: null
+  "Err": ""
 }
 ```
 
-Respond with a string error if an error occurred.
+Respond with a non-empty string error if an error occurred.
+
+### /GraphDriver.CreateReadWrite
+
+**Request**:
+```
+{
+  "ID": "46fe8644f2572fd1e505364f7581e0c9dbc7f14640bd1fb6ce97714fb6fc5187",
+  "Parent": "2cd9c322cb78a55e8212aa3ea8425a4180236d7106938ec921d0935a4b8ca142"
+  "MountLabel": ""
+}
+```
 
+Similar to `/GraphDriver.Create` but creates a read-write filesystem layer.
 
 ### /GraphDriver.Remove
 
@@ -82,11 +95,11 @@ Remove the filesystem layer with this given `ID`.
 **Response**:
 ```
 {
-  "Err: null
+  "Err": ""
 }
 ```
 
-Respond with a string error if an error occurred.
+Respond with a non-empty string error if an error occurred.
 
 ### /GraphDriver.Get
 
@@ -109,7 +122,7 @@ Get the mountpoint for the layered filesystem referred to by the given `ID`.
 ```
 
 Respond with the absolute path to the mounted layered filesystem.
-Respond with a string error if an error occurred.
+Respond with a non-empty string error if an error occurred.
 
 ### /GraphDriver.Put
 
@@ -126,11 +139,11 @@ filesystem layer.
 **Response**:
 ```
 {
-  "Err: null
+  "Err": ""
 }
 ```
 
-Respond with a string error if an error occurred.
+Respond with a non-empty string error if an error occurred.
 
 ### /GraphDriver.Exists
 
@@ -189,13 +202,13 @@ with the specified `ID`
 ```
 {
   "Metadata": {},
-  "Err": null
+  "Err": ""
 }
 ```
 
 Respond with a set of key/value pairs containing the low-level diagnostic
 information about the layered filesystem.
-Respond with a string error if an error occurred.
+Respond with a non-empty string error if an error occurred.
 
 ### /GraphDriver.Cleanup
 
@@ -210,11 +223,11 @@ unmounting all the layered file systems.
 **Response**:
 ```
 {
-  "Err: null
+  "Err": ""
 }
 ```
 
-Respond with a string error if an error occurred.
+Respond with a non-empty string error if an error occurred.
 
 
 ### /GraphDriver.Diff
@@ -252,7 +265,7 @@ Get a list of changes between the filesystem layers specified by the `ID` and
 ```
 {
   "Changes": [{}],
-  "Err": null
+  "Err": ""
 }
 ```
 
@@ -269,7 +282,7 @@ changed and `Kind` is an integer specifying the type of change that occurred:
 - 1 - Added
 - 2 - Deleted
 
-Respond with a string error if an error occurred.
+Respond with a non-empty string error if an error occurred.
 
 ### /GraphDriver.ApplyDiff
 
@@ -290,12 +303,12 @@ and `Parent`
 ```
 {
   "Size": 512366,
-  "Err": null
+  "Err": ""
 }
 ```
 
 Respond with the size of the new layer in bytes.
-Respond with a string error if an error occurred.
+Respond with a non-empty string error if an error occurred.
 
 ### /GraphDriver.DiffSize
 
@@ -313,9 +326,9 @@ Calculate the changes between the specified `ID`
 ```
 {
   "Size": 512366,
-  "Err": null
+  "Err": ""
 }
 ```
 
 Respond with the size changes between the specified `ID` and `Parent`
-Respond with a string error if an error occurred.
+Respond with a non-empty string error if an error occurred.