Add missing documentation of ChangeKind.MERGE_FIRST_PARENT_UPDATE

Change-Id: I83338f9e8c3db10004271056ba04ba00edf79639
diff --git a/Documentation/config-hooks.txt b/Documentation/config-hooks.txt
index 1d92b49..cde7b39 100644
--- a/Documentation/config-hooks.txt
+++ b/Documentation/config-hooks.txt
@@ -50,6 +50,8 @@
 
   TRIVIAL_REBASE;; Conflict-free merge between the new parent and the prior patch set.
 
+  MERGE_FIRST_PARENT_UPDATE;; Conflict-free change of first (left) parent of a merge commit.
+
   NO_CODE_CHANGE;; No code changed; same tree and same parent tree.
 
   NO_CHANGE;; No changes; same commit message, same tree and same parent tree.
diff --git a/Documentation/json.txt b/Documentation/json.txt
index db22212..ef40aee 100644
--- a/Documentation/json.txt
+++ b/Documentation/json.txt
@@ -116,6 +116,8 @@
 
   TRIVIAL_REBASE;; Conflict-free merge between the new parent and the prior patch set.
 
+  MERGE_FIRST_PARENT_UPDATE;; Conflict-free change of first (left) parent of a merge commit.
+
   NO_CODE_CHANGE;; No code changed; same tree and same parent tree.
 
   NO_CHANGE;; No changes; same commit message, same tree and same parent tree.