webview [nfc]: Further write up what we've learned about asset paths.
webview [nfc]: Further write up what we've learned about asset paths.

This follows up on b2b274c4b with some further things we learned in
the discussion on its PR #3677, and in a pair of related chat threads:
  https://chat.zulip.org/#narrow/stream/243-mobile-team/near/796432

Also add a couple more reference links, and adjust wording around
`file:///android_asset/` to avoid seeming to assert that it
corresponds to a directory really mounted somewhere in the filesystem.

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