[oemid] Expand documentation a little.

CHANGES

@@ -25,7 +25,8 @@ This release contains contributions from (alphabetically by first name):
 ## Modules ##
 
  - *oemid* is a new module for configuring OEM phase-0 (image pre-mastering,
-   or pre-deployment) things. It has limited functionality at the moment.
+   or pre-deployment) things. It has limited functionality at the moment,
+   writing only a single batch-identifier file.
  - All Python modules now bail out gracefully on (at least some) bad
    configurations, rather than raising an exception. The pre-release
    scripts now test for exceptions to avoid shipping modules with

src/modules/oemid/oemid.conf

@@ -4,4 +4,10 @@
 # This value is put into the text box as the **suggested**
 # OEM ID. If @@DATE@@ is included in the identifier, then
 # that is replaced by the current date in yyyy-MM-dd (ISO) format.
+#
+# it is ok for the identifier to be empty.
+#
+# The identifier is written to the file as UTF-8 (this will be no
+# different from ASCII, for most inputs) and followed by a newline.
+# If the identifier is empty, only a newline is written.
 batch-identifier: neon-@@DATE@@