Printing API: Update external documentation

Include chrome.printing html documentation to the list of available APIs.

Bug: 996785
Change-Id: I4868fb3737133c614297353c393d013f6492caff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2050378
Commit-Queue: Nikita Podguzov <nikitapodguzov@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743466}

chrome/common/extensions/docs/templates/public/extensions/printing.html

+{{+partials.standard_extensions_api api:apis.extensions.printing chrome_os_only:true/}}