Further improve iOS "Signing an App" and "Xcode Build Flags"

https://cordova.apache.org/docs/en/7.x/guide/platforms/ios/index.html#signing-an-app
https://cordova.apache.org/docs/en/7.x/guide/platforms/ios/index.html#xcode-build-flags

1. https://cordova.apache.org/docs/en/7.x/guide/platforms/ios/index.html#using-buildjson the newer examples also show build flags, that are only introduced in the headline below. That might be confusing.

2. The signing params still contain old stuff that is not to be used any more. This should be removed or made more clear that it is deprecated.

3. Build flags could use a little more explanation, examples and links to Apple docs maybe.