gn: Tweak precompiled_header docs.

Add an entry for precompiled_header_type, and document
GCC-style precompiled headers a bit.

BUG=none

Review-Url: https://codereview.chromium.org/2734523002
Cr-Original-Commit-Position: refs/heads/master@{#454685}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 698727682cc73ad3770d402395d23096f572d3f2
3 files changed
tree: a05df96e93417031a30bb65aa7252982d33b7269
  1. tools/