OPTIONAL field MAY be null

Signed-off-by: Lei Jitang <[email protected]>

Author: coolljt0725

config.md

@@ -111,7 +111,7 @@ Since the [configuration JSON](#image-json) that gets hashed references hashes o
    - **Entrypoint** *array of strings*, OPTIONAL
 
      A list of arguments to use as the command to execute when the container starts.
-     This value acts as a default and is replaced by an entrypoint specified when creating a container. This field MAY be null.
+     This value acts as a default and is replaced by an entrypoint specified when creating a container.
 
    - **Cmd** *array of strings*, OPTIONAL
 
@@ -121,7 +121,7 @@ Since the [configuration JSON](#image-json) that gets hashed references hashes o
 
    - **Volumes** *object*, OPTIONAL
 
-     A set of directories which should be created as data volumes in a container running this image. This field MAY be null.
+     A set of directories which should be created as data volumes in a container running this image.
      If a file or folder exists within the image with the same path as a data volume, that file or folder is replaced with the data volume and is never merged. **NOTE:** This JSON structure value is unusual because it is a direct JSON serialization of the Go type `map[string]struct{}` and is represented in JSON as an object mapping its keys to an empty object.
 
    - **WorkingDir** *string*, OPTIONAL
@@ -174,6 +174,7 @@ Any extra fields in the Image JSON struct are considered implementation specific
 
 Whitespace is OPTIONAL and implementations MAY have compact JSON with no whitespace.
 
+The OPTIONAL field MAY be null, it means this field present but having an empty/null value.
 ## Example
 
 Here is an example image configuration JSON document: