ShowDoc: display "current context" message

Consider the following example:

  [1] pry(main)> show-doc
  Error: No docs found for:
  [2] pry(main)>

The message is ambiguous. This commit adds "current context" label when
`obj_name` return `nil`.

  [1] pry(main)> show-doc
  Error: No docs found for: current context
  [2] pry(main)>