webview: Add doc comment on "scroll"-on-load hack.
webview: Add doc comment on "scroll"-on-load hack.

See 37dfd5a1e "webview: Send a 'scroll' event on load", which
introduced this hack.

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile