[fstab] Document the remapping of fs names

- Shuffle the code a little so it's clear where the remapping
  happens, and explain why we look for "swap" in fstab and
  "linuxswap" elsewhere.

src/modules/fstab/main.py

@@ -232,14 +232,21 @@ class FstabGenerator(object):
                 self.print_fstab_line(dct, file=fstab_file)
 
     def generate_fstab_line_info(self, partition):
-        """ Generates information for each fstab entry. """
+        """
+        Generates information (a dictionary of fstab-fields)
+        for the given @p partition.
+        """
+        # Some "fs" names need special handling in /etc/fstab, so remap them.
         filesystem = partition["fs"].lower()
+        filesystem = FS_MAP.get(filesystem, filesystem)
         has_luks = "luksMapperName" in partition
         mount_point = partition["mountPoint"]
         disk_name = disk_name_for_partition(partition)
         is_ssd = disk_name in self.ssd_disks
-        filesystem = FS_MAP.get(filesystem, filesystem)
 
+        # Swap partitions are called "linuxswap" by parted.
+        # That "fs" is visible in GS, but that gets mapped
+        # to "swap", above, because that's the spelling needed in /etc/fstab
         if not mount_point and not filesystem == "swap":
             return None
         if not mount_point: