help: document the ''HG_TXNID'' environment variable during hooks

We forgot to document the new "transaction ID" mechanism.

mercurial/help/config.txt

@@ -810,8 +810,9 b' variables it is passed are listed with n'
 
 ``pretxnopen``
   Run before any new repository transaction is open. The reason for the
-  transaction will be in ``$HG_TXNNAME``. A non-zero status will
-  prevent the transaction from being opened.
+  transaction will be in ``$HG_TXNNAME`` and a unique identifier for the
+  transaction will be in ``HG_TXNID``. A non-zero status will prevent the
+  transaction from being opened.
 
 ``pretxnclose``
   Run right before the transaction is actually finalized. Any
@@ -819,11 +820,12 b' variables it is passed are listed with n'
   validate the transaction content or change it. Exit status 0 allows
   the commit to proceed. Non-zero status will cause the transaction to
   be rolled back. The reason for the transaction opening will be in
-  ``$HG_TXNNAME``. The rest of the available data will vary according
-  the transaction type. New changesets will add
-  ``$HG_NODE`` (id of the first added changeset), ``$HG_URL`` and
-  ``$HG_SOURCE`` variables, bookmarks and phases changes will set
-  ``HG_BOOKMARK_MOVED`` and ``HG_PHASES_MOVED`` to ``1``, etc.
+  ``$HG_TXNNAME`` and a unique identifier for the transaction will be in
+  ``HG_TXNID``. The rest of the available data will vary according the
+  transaction type. New changesets will add ``$HG_NODE`` (id of the
+  first added changeset), ``$HG_URL`` and ``$HG_SOURCE`` variables,
+  bookmarks and phases changes will set ``HG_BOOKMARK_MOVED`` and
+  ``HG_PHASES_MOVED`` to ``1``, etc.
 
 ``txnclose``
   Run after any repository transaction has been commited. At this