Update documentation for DISPLAY_DECORATION

After some discussion, we've decided to always set the ScreenDecorations
to DISPLAY_DECORATION. HWC can decide, based on the format, whether to
truly treat it as DISPLAY_DECORATION, or to change to DEVICE
composition.

Update the documentation to reflect this change.

See also I1da3199bacf5877e88876249c60c0a33fcd44c1e.

Bug: 193170859
Test: NA
Change-Id: I1fae74b360d1b9d79fc4afeab6f71089d4c8b7e5