docs: Update private headers list

docs/reference/gtk/meson.build

@@ -6,17 +6,24 @@ private_headers = [
   'gdkpixbufutilsprivate.h',
   'gtkaccelgroupprivate.h',
   'gtkaccelmapprivate.h',
+  'gtkaccessibleattributesetprivate.h',
+  'gtkaccessibleattributevalueprivate.h',
+  'gtkaccessibleprivate.h',
+  'gtkaccessiblevalueprivate.h',
   'gtkactionhelperprivate.h',
   'gtkactionmuxerprivate.h',
+  'gtkactionobservableprivate.h',
+  'gtkactionobserverprivate.h',
   'gtkadjustmentprivate.h',
   'gtkallocatedbitmaskprivate.h',
   'gtkappchooserprivate.h',
   'gtkapplicationaccelsprivate.h',
   'gtkapplicationprivate.h',
-  'gtkarrayimplprivate.h',
+  'gtkatcontextprivate.h',
   'gtkbindingsprivate.h',
   'gtkbitmaskprivate.h',
   'gtkbuilderprivate.h',
+  'gtkbuilderscopeprivate.h',
   'gtkbuttonprivate.h',
   'gtkcellareaboxcontextprivate.h',
   'gtkcheckbuttonprivate.h',
@@ -45,6 +52,7 @@ private_headers = [
   'gtkcssarrayvalueprivate.h',
   'gtkcssbgsizevalueprivate.h',
   'gtkcssbordervalueprivate.h',
+  'gtkcssboxesimplprivate.h',
   'gtkcssboxesprivate.h',
   'gtkcsscalcvalueprivate.h',
   'gtkcsscolorvalueprivate.h',