Revert of Revert of Some webview documentation polish fixes. (patchset #1 id:1 of...

Revert of Revert of Some webview documentation polish fixes. (patchset #1 id:1 of https://codereview.chromium.org/1116003004/)

Reason for revert:
This CL is unrelated to the flaky tests (it is literally just updating documentation with no functional changes). There is a fix coming soon (from hanxi@) for the flaky tests.

Original issue's description:
> Revert of Some webview documentation polish fixes. (patchset #2 id:20001 of https://codereview.chromium.org/1109393008/)
>
> Reason for revert:
> Suspect cause of http://build.chromium.org/p/chromium.chromiumos/builders/Linux%20ChromiumOS%20Tests%20%28dbg%29%281%29/builds/1679
>
> Also, references to  WebRequestEventInteface in the code:
>
> chrome/common/extensions/docs/server2/known_broken_links.json
> ui/file_manager/externs/webview_tag.js
>
> Original issue's description:
> > Some webview documentation polish fixes.
> >
> > BUG=481594
> >
> > Committed: https://crrev.com/8851f9f79ef2e15130eec205a66c277f810aafc5
> > Cr-Commit-Position: refs/heads/master@{#327736}
>
> TBR=fsamuel@chromium.org,paulmeyer@chromium.org
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=481594
>
> Committed: https://crrev.com/68c15d61f7eb1821b8df3b4ecb0c5985c70114a3
> Cr-Commit-Position: refs/heads/master@{#327777}

TBR=fsamuel@chromium.org,stevenjb@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=481594

Review URL: https://codereview.chromium.org/1121573002

Cr-Commit-Position: refs/heads/master@{#327784}