Icon docs are wrong with respect to iOS

Our documentation at https://cordova.apache.org/docs/en/latest/config_ref/images.html#configuring-icons-in-the-cli states:

The following configuration can be used to define a single default icon which will be used for all platforms.

<icon src="res/icon.png" />

For each platform, you can also define a pixel-perfect icon set to fit different screen resolutions.

For iOS, this is only partially true. You can compile an app by specifying a single icon if and only if that icon matches one of Apple's required sizes, but a single icon won't meet Apple's guidelines, and generates warnings when compiling the asset catalog.

We should either respect the docs which would mean generating any missing Apple icons from the <icon> tag, OR fix up the docs to state that iOS requires correctly-sized icons.