Word the section about libc++ more strongly.

The libc++ support is still pretty awful. We need to make it clear that
this is something people should only be using if they are confident in
their testing.

The test details are out of date (and I don't believe they were ever
correct), and always will be since this is under active development.
Drop the specifics and just list some big KIs. Also mention that users
should check the changelog for other KIs.

Bug: http://b/21711315
Change-Id: I42f0c8ab963d7f77462b89335cf0d66cf6035388