Update GN docs about Visual Studio generators

BUG=

Review URL: https://codereview.chromium.org/1750523002

Cr-Original-Commit-Position: refs/heads/master@{#378419}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: eca97f87e275a7c9c5b7f13a65ff8635f0821d46