docs: read the xemu hdd name as a label

emulators/xemu.yml

@@ -171,20 +171,19 @@ files:
     required: true
     installed_on: [batocera, emudeck, recalbox, retrodeck, rocknix]
     config_key: sys.files.hdd_path
+    found_by: content
     description: "Xbox hard disk image, 8 GB with the retail partition layout"
     note: >-
       Attached as the IDE drive at index 0 with locked=on when the path is set
       and the file opens. An unreadable file only queues an error and an empty
-      path attaches no drive. Snapshots are stored inside the image. The
-      project publishes a formatted image carrying its own unsigned dashboard
-      and empty TDATA and UDATA folders.
-      The xemu release does not carry the image. Batocera and Recalbox
-      package the 1.0 xemu-hdd-image and copy it into the saves folder at
-      launch when none is there, RetroDECK copies its packaged copy into the
-      BIOS folder during setup, and EmuDeck downloads it while setting xemu
-      up. ROCKNIX packages the same image and unzips it at launch into
-      bios/xemu/hdd/, the path its xemu.toml gives hdd_path.
-    source_ref: "system/vl.c:3060-3074, ui/xemu-snapshots.c:156"
+      path attaches no drive. Snapshots are stored inside the image.
+      hdd_path has no default, so the name is a label: it is the file of
+      the formatted image the project publishes apart from the xemu release
+      (xemu-hdd-image 1.0, xbox_hdd.qcow2.zip), which carries its own
+      unsigned dashboard and empty TDATA and UDATA folders. Batocera,
+      Recalbox, RetroDECK, EmuDeck and ROCKNIX install that image themselves
+      and point hdd_path at it.
+    source_ref: "system/vl.c:3060-3074, ui/xemu-snapshots.c:156, config_spec.yml:362-366"
 
   - name: xbox_hdd.qcow2
     mode: libretro