The current documentation shows a debug build.
When you attempt that an assert is triggered suggesting
a release build.  Update the documentation to reflect that.

TBR=kjellander@chromium.org
BUG=libyuv:731
TEST=gn gen out/Deb^C "--args=is_debug=true is_msan=true"

Change-Id: Ib78e3a7859aa391ac359fa38234aaef0554d0df4
Reviewed-on: https://chromium-review.googlesource.com/663882Reviewed-by: Cheng Wang <wangcheng@google.com>