content: Clarify use of ion-input with counter inside ion-list

[averyjohnston]

URL

https://ionicframework.com/docs/api/input

Issue Description

While we do mention that ion-input with counter="true" and other metadata should not be used in an ion-item, it's down in the migration guide and is not easy to find. There are a couple things we should do here:

1. Add a note to the Input Counter section (https://ionicframework.com/docs/api/input#input-counter) that inputs should not be used with ion-item in this scenario.
2. Add example code snippets (possibly a new playground) showing how to use counter inputs in a list with non-counter inputs, and have all of them show dividing borders. It looks like the easiest way is to wrap the counter inputs in <div class="ion-padding-start"></div> to line up the horizontal padding; example: https://codepen.io/amandaejohnston/pen/bGQJydP