AW docs: clarify GN args

No change to logic, only docs.

This clarifies in the quick-start that "setting up the build" really
means configuring GN args. This should be obvious to most readers, but
it's probably worth being explicit since this stuff could easily be
misinterpreted as setting shell variables. It also clarifies that the
doc expects folks to use `out/Default` as their out folder.

Bug: 918221
Test: tools/md_browser/md_browser.py
Test: Upload to gerrit > open file > click "gitiles"
Change-Id: I533eb37438135012229089ec53150b1b99619b8c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1639038
Auto-Submit: Nate Fischer <ntfschr@chromium.org>
Commit-Queue: Shimi Zhang <ctzsm@chromium.org>
Reviewed-by: Shimi Zhang <ctzsm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665319}