Clarify comments in message properties files (and fix grammar)

Bug: 438261
Change-Id: I7b98475886ef789ae7635d8c9e008fc1aa00d534
Signed-off-by: Robin Stocker <robin@nibor.org>

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

@@ -1,6 +1,6 @@
 #
-# These text are in general processed using java.text.MessageFormat.
+# Messages with format elements ({0}) are processed using java.text.MessageFormat.
-# exceptions are the usage_* and meta_-messages which are processed through
+# 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

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

@@ -1,5 +1,5 @@
 #
-# These text are processed using java.text.MessageFormat.
+# Messages with format elements ({0}) are processed using java.text.MessageFormat.
 #
 abbreviationLengthMustBeNonNegative=Abbreviation length must not be negative.
 abortingRebase=Aborting rebase: resetting to {0}