docs: show the FAT32 setting in the compose file

The option table listed it, but compose.yaml is what actually gets copied into
a TrueNAS Custom App, so an option absent from there is an option nobody finds.

Quoted deliberately: an unquoted yes or true is a YAML 1.1 boolean, and compose
rejects a boolean as an environment value outright.

compose.yaml

@@ -22,6 +22,15 @@ services:
       # Concurrent encodes; defaults to the CPU count. Lower it to leave the
       # NAS responsive during the first full pass.
       # MUSIC_MIRROR_JOBS: "4"
+      # Name files so a FAT32 device will take them, for copying to a Rockbox
+      # player. Quoted, because an unquoted yes or true is a YAML boolean and
+      # compose rejects a boolean here. Accepts 1, true or yes.
+      #
+      # Turning this on renames every file whose name held a reserved
+      # character. They are moved, not re-encoded, so the first pass after
+      # enabling it is quick -- but it is a one-way change to every such path,
+      # so decide before running it rather than after.
+      MUSIC_MIRROR_FAT32_SAFE: "true"
     volumes:
       - /mnt/tank/media/music:/music:ro
       - /mnt/tank/media/music-mp3:/mirror