change the labels and documentation around 'ipa' and 'dsym' plugin fields

due to changes to the _documentation_ on the HockeyApp site, i propose
that the plugin describe all the file types that can be uploaded in the
'ipa' and 'dsym' fields of the HockeyApp 'Upload New Versions' API.

http://support.hockeyapp.net/kb/api/api-upload-new-versions

* ipa - optional (required, if dsym is not specified for iOS or Mac),
      file data of the .ipa for iOS, .app.zip for Mac OS X, or .apk
      file for Android
* dsym - optional, file data of the .dSYM.zip file (iOS and Mac) or
       mapping.txt (Android)

see also this discussion:

http://support.hockeyapp.net/discussions/questions/1538-jenkins-ci-is-there-an-upload-apiplugin-for-macos-apps