[fstab] Tighten up the *efiMountOptions* documentation

src/modules/fstab/fstab.conf

@@ -12,9 +12,9 @@ mountOptions:
     default: defaults,noatime
     btrfs: defaults,noatime,space_cache,autodefrag
 
-# Mount options to use for the EFI System Partition. If not defined, it
+# Mount options to use for the EFI System Partition. If not defined, the
-# takes either the value of *vfat*, or if also missing, *default* from
+# *mountOptions* for *vfat* are used, or if that is not set either,
-# *mountOptions*
+# *default* from *mountOptions*.
 efiMountOptions: umask=0077
 
 # If a filesystem is on an SSD, add the following options. If a specific