Document Xcode Cordova build pitfall

Coming from the symptom from :

> I get the same error.
>
> 

with explanation of the pitfall from :

> it looks like your Xcode GUI is trying to just build the Cordova subproject, which is not really tested or supported by us. Please click on this control and use it to select your application build project:
>
>
>
> I suspect that this would affect other users. I think this should be documented as a pitfall.

The screenshots may be outdated but I think the pitfall is self-evident and needs to be documented. I think this is not specific to Ionic.

Read the linked cordova-ios issue comments first to confirm the Xcode project-selection pitfall and its wording. Then locate the appropriate Cordova iOS documentation page and document that Xcode should build the application project rather than the Cordova subproject, checking whether the referenced screenshots are still current.

Written by the indexing model from the issue text.