[ledger] Document sending multiple times the same key in Merge

BUG=LE-653 #done
TESTED=Documentation change only

Change-Id: I2505f34dde047da3eb663df0049b0d9df64e7a4d
diff --git a/public/fidl/fuchsia.ledger/ledger.fidl b/public/fidl/fuchsia.ledger/ledger.fidl
index 0d16804..e393885 100644
--- a/public/fidl/fuchsia.ledger/ledger.fidl
+++ b/public/fidl/fuchsia.ledger/ledger.fidl
@@ -499,6 +499,9 @@
     /// |Merge()| calls fails, i.e. |status| is not |OK|, all following calls will
     /// fail with the same error.
     ///
+    /// If a key/value pair is sent multiple times in one or several |Merge()|
+    /// call, only the last pair is taken into account.
+    ///
     /// For all keys for which no merged value has been set (either here or
     /// through |MergeNonConflictingEntries()| below), the left value will be
     /// used. It is thus not necessary to send a MergedValue with a |LEFT| value