Pause and PLR refinements

- Move `pause_print` argument `unload_length` after `show_lcd` so it's next to `DXC_ARGS`.
- Tweak the position and conditions of PLR save in `resume_print`.
- Add `Nozzle::park_mode_0_height` accessor to get the raised Z height.
- Remove extraneous `recovery.save` from `dwin.cpp`.
- Move PLR `info.volumetric...` to `flag`.
- Remove some G-code spaces in PLR code
- Document `pause.h` function declarations.

Marlin/src/feature/powerloss.h

@@ -70,7 +70,6 @@ typedef struct {
   #endif
 
   #if DISABLED(NO_VOLUMETRICS)
-    bool volumetric_enabled;
     float filament_size[EXTRUDERS];
   #endif
 
@@ -116,7 +115,10 @@ typedef struct {
     bool dryrun:1;                // M111 S8
     bool allow_cold_extrusion:1;  // M302 P1
     #if ENABLED(HAS_LEVELING)
-      bool leveling:1;
+      bool leveling:1;            // M420 S
+    #endif
+    #if DISABLED(NO_VOLUMETRICS)
+      bool volumetric_enabled:1;  // M200 S D
     #endif
   } flag;