[partition] Document interactions between defaultFS and layout

src/modules/partition/partition.conf

@@ -215,7 +215,12 @@ availableFileSystemTypes:  ["ext4", "btrfs", "f2fs"]
 #   - uuid: partition uuid (optional parameter; gpt only; requires KPMCore >= 4.2.0)
 #   - type: partition type (optional parameter; gpt only; requires KPMCore >= 4.2.0)
 #   - attributes: partition attributes (optional parameter; gpt only; requires KPMCore >= 4.2.0)
-#   - filesystem: filesystem type (optional parameter; fs not created if "unformatted" or unset)
+#   - filesystem: filesystem type (optional parameter)
+#       - if not set at all, treat as "unformatted"
+#       - if "unformatted", no filesystem will be created
+#       - if "unknown" (or an unknown FS name, like "elephant") then the
+#         default filesystem type, or the user's choice, will be applied instead
+#         of "unknown" (e.g. the user might pick ext4, or xfs).
 #   - mountPoint: partition mount point (optional parameter; not mounted if unset)
 #   - size: partition size in bytes (append 'K', 'M' or 'G' for KiB, MiB or GiB)
 #           or