What problem does this feature solve?

Per the conversation in issues #4069 and my own experience, the current documentation on icons for the PWA plugin does not clearly explain the difference between pwa.iconPaths and pwa.manifestOptions.icons. In fact pwa.manifestOptions.icons isn't actually mentioned in the docs.

What does the proposed API look like?

Simply adding a note to the pwa.iconPaths section explaining what these icon paths do and adding pwa.manifestOptions.icons to the pwa.manifestOptions section should solve most peoples confusion.