📚 Fix #response documentation error

`Net::IMAP#response` only stores non-nil `#data`, not the entire object.

Author: nevans

lib/net/imap.rb

@@ -511,10 +511,8 @@ module Net
  #
  # - #greeting: The server's initial untagged response, which can indicate a
  # pre-authenticated connection.
- # - #responses: The untagged responses, as a hash. Keys are the untagged
- # response type (e.g. "OK", "FETCH", "FLAGS") and response code (e.g.
- # "ALERT", "UIDVALIDITY", "UIDNEXT", "TRYCREATE", etc). Values are arrays
- # of UntaggedResponse or ResponseCode.
+ # - #responses: A hash with arrays of unhandled <em>non-+nil+</em>
+ # UntaggedResponse and ResponseCode +#data+, keyed by +#name+.
  # - #add_response_handler: Add a block to be called inside the receiver thread
  # with every server response.
  # - #remove_response_handler: Remove a previously added response handler.
@@ -712,7 +710,9 @@ class IMAP < Protocol
  # Returns the initial greeting the server, an UntaggedResponse.
  attr_reader :greeting
 
- # Returns recorded untagged responses.
+ # Returns a hash with arrays of unhandled <em>non-+nil+</em>
+ # UntaggedResponse#data keyed by UntaggedResponse#name, and
+ # ResponseCode#data keyed by ResponseCode#name.
  #
  # For example:
  #