docs: Add note about missing recording functionality in Live Channels

TV Recording API is available in N Preview, however the Google
Live Channels app is still being updated to add UI for recording.
Until the Live Channels app has been updated, devs won't be able
to test the full user experience for recording. We're adding a
note to let devs know.

See plan/discussion at: https://goto.google.com/wiceq

See CL comments for staged URL

Bug: 27920114
Change-Id: I02490849c054fbd53cb9011f2fc8c908dc524f51