iframe support or lack thereof

**Issue Type**
[ ] Bug
[ ] Feature Request
[x] testing
[x] documentation

**Priority**
[x] Minor
[ ] Major
[ ] Critical
[ ] Blocker

**Environment**
* OS: all
* Browser: multiple

**Description**

- iframe may not work on iOS WKWebView as discussed in iOS WKWebView blog post (#867)
- uncertain about iframe support on Windows

Start by reading the iOS WKWebView blog post referenced in #867, then investigate iframe behavior on iOS WKWebView and Windows. The work is complete when the documentation clearly states whether iframes are supported on each platform and records any relevant limitations.

Written by the indexing model from the issue text.