[grubcfg] Update documentation of config file

- add some more general description
 - document new-to-implement *keepDistributor* flag

SEE #1201

src/modules/grubcfg/grubcfg.conf

@@ -1,9 +1,12 @@
+# Create, overwrite or update /etc/default/grub in the target system.
+#
 # 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_DISTRIBUTOR,   branding module, *bootloaderEntryName* (this
+#     string is sanitized, and see also setting *keepDistributor*)
 #   - GRUB_ENABLE_CRYPTODISK, based on the presence of filesystems
 #     that use LUKS
 #   - GRUB_CMDLINE_LINUX_DEFAULT, adding LUKS setup and plymouth
@@ -14,6 +17,12 @@
 # already existed. If set to false, edits the existing file instead.
 overwrite: false
 
+# If set to true, an **existing** setting for GRUB_DISTRIBUTOR is
+# kept, not updated to the *bootloaderEntryName* from the branding file.
+# Use this if the GRUB_DISTRIBUTOR setting in the file is "smart" in
+# some way (e.g. uses shell-command substitution).
+keepDistributor: false
+
 # Default entries to write to /etc/default/grub if it does not exist yet or if
 # we are overwriting it.
 #