[webview_flutter] Update webview_flutter documentation

[NickalasB]

This is a follow-up PR to #3325. After updating our repo to use version 3.2.0, I realized that in order to actually disable zoom on iOS, we have to set javascriptMode: JavascriptMode.unrestricted as iOS uses javascript to disable zoom.

This PR just updates the documentation to reflect this.

List which issues are fixed by this PR. You must list at least one issue.
flutter/flutter#75196

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[NickalasB]

@stuartmorgan @bparrishMines @mvanbeusekom This slipped by me as the example app already has javascriptMode: JavascriptMode.unrestricted. Feels like it's a necessary note in the event someone actually wants to disable zoom on iOS.

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[stuartmorgan-g]

LGTM

[mvanbeusekom]

LGTM