Reland "Update documentation for DISPLAY_DECORATION"

The first time this topic landed, it resulted in b/212402133. We avoid
running into this bug with Ib11d46439db57b90486bad07dd90f2cf0822182a.

Remove references to 8 bit alpha.

Original commit message:

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: Iaf5385c38f6827e07b91a9a0a8769fb7a125e847