Requires teaching the jumbo scripts about .S files.

Also fix some minor formatting issues in the jumbo docs.

On my Mac laptop, for release component builds:
* increases by 536% (from 3.3s to 21s) incremental build times
  of 'base', after touching base/strings/string16.cc.
* reduces by 65% (from 91s to 32s) full build times of 'base'

So this looks like something that's useful for full builds and
for playing with compiler flags (which requires full builds),
but probably not something for daily development.  I suppose
that's true of jumbo builds in general.

Bug: 775547
Change-Id: I9b5157682ee463e5d60499f4dc8815283a127729
Reviewed-on: https://chromium-review.googlesource.com/721139


Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: oysteine <oysteine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509524}