message: Add `null`, `unbound` to schema for keybindings
  ## Summary of the Pull Request
  Add `null`, `unbound` to schema for keybindings
  ## PR Checklist
  * [x] Closes
  * [x] I work here
  * [n/a] Tests added/passed
  * [x] Requires documentation to be updated
diff: "splitPane",
  "switchToTab",
  "toggleFullscreen",
  - "find"
  + "find",
  + "unbound"
  ],
  "type": "string"
  },
  { "$ref": "#/definitions/SwitchToTabAction" },
  { "$ref": "#/definitions/MoveFocusAction" },
  { "$ref": "#/definitions/ResizePaneAction" },
  - { "$ref": "#/definitions/SplitPaneAction" }
  + { "$ref": "#/definitions/SplitPaneAction" },
  + { "type": "null" }
  ]
  },
  "keys": {
  