Add a comment to the message properties files on how they are processed

Change-Id: I073f0c2c0729e6a5d3f1834203f0cfeb4c462eda

org.eclipse.jgit.pgm/resources/org/eclipse/jgit/pgm/internal/CLIText.properties

@@ -1,3 +1,8 @@
+#
+# These text are in general processed using java.text.MessageFormat.
+# exceptions are the usage_* and meta_-messages which are processed through
+# annotations in Args4j.
+#
 # default meta variable defined in the org.kohsuke.args4j.spi.StringOptionHandler
 VAL=VAL
 # default meta variable defined in the org.kohsuke.args4j.spi.StopOptionHandler

org.eclipse.jgit/resources/org/eclipse/jgit/internal/JGitText.properties

@@ -1,3 +1,6 @@
+#
+# These text are processed using java.text.MessageFormat.
+#
 abbreviationLengthMustBeNonNegative=Abbreviation length must not be negative.
 abortingRebase=Aborting rebase: resetting to {0}
 abortingRebaseFailed=Could not abort rebase