[modules] Documentation for fstab, grubcfg, mkinitcpio

- These modules were entirely documented as "use the source",
 - The sources aren't terribly clear either.

src/modules/fstab/fstab.conf

@@ -1,13 +1,28 @@
+# Creates /etc/fstab and /etc/crypttab in the target system.
+# Also creates mount points for all the filesystems.
+#
+# When creating fstab entries for a filesystem, this module
+# uses the options for the filesystem type to write to the
+# options field of the file.
 ---
+# Mount options to use for all filesystems. If a specific filesystem
+# is listed here, use those options, otherwise use the *default*
+# options from this mapping.
 mountOptions:
     default: defaults,noatime
     btrfs: defaults,noatime,space_cache,autodefrag
+
+# If a filesystem is on an SSD, add the following options. If a specific
+# filesystem is listed here, use those options, otherwise no additional
+# options are set (i.e. there is no *default* like in *mountOptions*).
 ssdExtraMountOptions:
     ext4: discard
     jfs: discard
     xfs: discard
     swap: discard
     btrfs: discard,compress=lzo
+
+# Additional options added to each line in /etc/crypttab
 crypttabOptions: luks
 # For Debian and Debian-based distributions, change the above line to:
 # crypttabOptions: luks,keyscript=/bin/cat

src/modules/grubcfg/grubcfg.conf

@@ -1,11 +1,22 @@
+# Write lines to /etc/default/grub (in the target system) based
+# on calculated values and the values set in the *defaults* key
+# in this configuration file.
+#
+# Calculated values are:
+#   - GRUB_DISTRIBUTOR,   branding module, *bootloaderEntryName*
+#   - GRUB_ENABLE_CRYPTODISK, based on the presence of filesystems
+#     that use LUKS
+#   - GRUB_CMDLINE_LINUX_DEFAULT, adding LUKS setup and plymouth
+#     support to the kernel.
+
 ---
 # If set to true, always creates /etc/default/grub from scratch even if the file
 # already existed. If set to false, edits the existing file instead.
 overwrite: false
 
 # Default entries to write to /etc/default/grub if it does not exist yet or if
-# we are overwriting it. Note that in addition, GRUB_CMDLINE_LINUX_DEFAULT and
+# we are overwriting it.
-# GRUB_DISTRIBUTOR will always be written, with automatically detected values.
+#
 defaults:
     GRUB_TIMEOUT: 5
     GRUB_DEFAULT: "saved"

src/modules/initcpio/initcpio.conf

@@ -1,2 +1,3 @@
+# Run mkinitcpio(8) with the given preset value
 ---
 kernel: linux312