Add chrome.loginState documentation

Bug: 980460
Change-Id: I1a6695afd602b41182216d249fbbb2c1cffe61ad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2090417Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Alexander Hendrich <hendrich@chromium.org>
Commit-Queue: Anina Koehler <aninak@google.com>
Cr-Commit-Position: refs/heads/master@{#748582}

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

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