docs(ios): document new InspectableWebview setting

[BBosman]

Platforms affected

iOS

Motivation and Context

Adds documentation for the changes introduced in apache/cordova-ios#1300 and apache/cordova-plugin-inappbrowser#995.

Description

Adds documentation for the InspectableWebview preference setting.

Testing

English isn't my native language, so please feel free to suggest better wording.

Checklist

• I've run the tests to see all new and existing tests pass
• I added automated test coverage as appropriate for this change
• Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• I've updated the documentation if necessary