Cut the drm package comments back to what is not evident from the code

internal/drm/drm.go

@@ -1,7 +1,4 @@
-// Package drm is the kernel mode-setting ABI, shared by the panel this draws on
-// and the grabber that reads it back out. These structs are a layout contract,
-// so a field added or reordered in one copy and not the other would silently
-// misread the kernel: there is deliberately only one copy.
+// Package drm is the kernel mode-setting ABI.
 package drm
 
 import (
@@ -93,9 +90,8 @@ type ModeCrtcPageFlip struct {
 	UserData                      uint64
 }
 
-// WaitVblank has only Type and Sequence ever set, and those sit at the same
-// offsets in both arms of the kernel's union. The request below carries the
-// union's own size rather than this struct's, which is larger.
+// WaitVblank is larger than the kernel union it mirrors, so WaitVblankIO
+// carries the union's size and only Type and Sequence are at valid offsets.
 type WaitVblank struct {
 	Type     uint32
 	Sequence uint32
@@ -122,7 +118,6 @@ var (
 	MapDumb      = IOWR(0xb3, unsafe.Sizeof(ModeMapDumb{}))
 )
 
-// What ADDFB means by 32 bits per pixel and 24 bits of colour.
 const (
 	redShift   = 16
 	greenShift = 8
@@ -137,9 +132,8 @@ func Unpack(v uint32) (r, g, b uint8) {
 	return uint8(v >> redShift), uint8(v >> greenShift), uint8(v >> blueShift)
 }
 
-// Offset turns the quarter turn between the portrait canvas and the landscape
-// panel, putting logical top on the panel's right edge. Drawing and reading
-// back have to agree on this exactly.
+// Offset maps the portrait canvas onto the landscape panel, a quarter turn that
+// puts logical top on the panel's right edge.
 func Offset(x, y, stride, panelW int) int {
 	return x*stride + (panelW-1-y)*4
 }