Document defaultFileSystemType

- this applies to new partitions; existing documentation erroneously
   said this happens with Replace as well,
 - follow up on Andrius manual-partition PR with documentation that
   manual mode doesn't switch to this FS when editing existing partitions.

src/modules/partition/partition.conf

@@ -26,10 +26,16 @@ drawNestedPartitions:   false
 # Show/hide partition labels on manual partitioning page.
 alwaysShowPartitionLabels: true
 
-# Default filesystem type, pre-selected in the "Create Partition" dialog
+# Default filesystem type, used when a "new" partition is made.
-# in the manual partitioning mode.
+#
-# The filesystem type selected here is also used for automated install
+# When replacing a partition, the existing filesystem inside the
-# modes (Erase, Replace and Alongside).
+# partition is retained. In other cases, e.g. Erase and Alongside,
+# as well as when using manual partitioning and creating a new
+# partition, this filesystem type is pre-selected. Note that
+# editing a partition in manual-creation mode will not automatically
+# change the filesystem type to this default value -- it is not
+# creating a new partition.
+#
 # Suggested values: ext2, ext3, ext4, reiser, xfs, jfs, btrfs
 # If nothing is specified, Calamares defaults to "ext4".
 defaultFileSystemType:  "ext4"