feat: added support OutputResult in ExportAssetsResponse. docs: updated existing docs.

Clients receive detailed OutputResult in ExportAssetsResponse.

PiperOrigin-RevId: 326582222

Source-Author: Google APIs <noreply@google.com>
Source-Date: Thu Aug 13 20:45:57 2020 -0700
Source-Repo: googleapis/googleapis
Source-Sha: 3a54e988edcbdef1e47c6ac19d3074a87214f667
Source-Link: googleapis/googleapis@3a54e98

Author: yoshi-automation