fuchsia: Update docs with note about building all

Bug: 808287
TBR: jeremymanson
Change-Id: I252d13864e5749c0e9c15bf2e8b3771d2fdd3683
Reviewed-on: https://chromium-review.googlesource.com/c/1324106
Reviewed-by: Scott Graham <scottmg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606137}
1 file changed