Docs: expand comments in versioning code

Help ensure we update all necessary areas when version bumping.

source/blender/blenloader/intern/versioning_280.c

@@ -4266,8 +4266,18 @@ void blo_do_versions_280(FileData *fd, Library *UNUSED(lib), Main *bmain)
     }
   }
 
+  /**
+   * Versioning code until next subversion bump goes here.
+   *
+   * \note Be sure to check when bumping the version:
+   * - "versioning_userdef.c", #BLO_version_defaults_userpref_blend
+   * - "versioning_userdef.c", #do_versions_theme
+   *
+   * \note Keep this message at the bottom of the function.
+   */
   {
-    /* Versioning code until next subversion bump goes here. */
+    /* Keep this block, even when empty. */
+
     /* Cloth internal springs */
     for (Object *ob = bmain->objects.first; ob; ob = ob->id.next) {
       for (ModifierData *md = ob->modifiers.first; md; md = md->next) {

source/blender/blenloader/intern/versioning_userdef.c

@@ -164,10 +164,17 @@ static void do_versions_theme(const UserDef *userdef, bTheme *btheme)
     FROM_DEFAULT_V4_UCHAR(space_view3d.face_front);
   }
 
-  /* Versioning code until next subversion bump goes here.
-   * Note: be sure to check "versioning_{BLENDER_VERSION}.c" when bumping the version.
-   * Note: keep this message at the bottom of the function. */
+  /**
+   * Versioning code until next subversion bump goes here.
+   *
+   * \note Be sure to check when bumping the version:
+   * - #BLO_version_defaults_userpref_blend in this file.
+   * - "versioning_{BLENDER_VERSION}.c"
+   *
+   * \note Keep this message at the bottom of the function.
+   */
   {
+    /* Keep this block, even when empty. */
   }
 
 #undef FROM_DEFAULT_V4_UCHAR
@@ -686,10 +693,16 @@ void BLO_version_defaults_userpref_blend(Main *bmain, UserDef *userdef)
   }
 
   /**
-   * Include next version bump.
+   * Versioning code until next subversion bump goes here.
+   *
+   * \note Be sure to check when bumping the version:
+   * - #do_versions_theme in this file.
+   * - "versioning_{BLENDER_VERSION}.c"
+   *
+   * \note Keep this message at the bottom of the function.
    */
   {
-    /* pass */
+    /* Keep this block, even when empty. */
   }
 
   if (userdef->pixelsize == 0.0f) {