config: fix subjects flag, add example for editing global config

Created by: bobheadxi

Currently, providing a -subject flag to src config does not apply it.

+ src config edit -subject=U2l0ZToic2l0ZSI= -property=notices '-value=[]'
GraphQL errors: 1 error occurred:
        * {
  "locations": [
    {
      "column": 39,
      "line": 2
    }
  ],
  "message": "Variable \"subject\" has invalid value null.\nExpected type \"ID!\", found null."
}

Also, the docs state that this command can edit global config, but does not imply how, so I added an example with a workaround - let me know if there's a proper way to do this!