[PATCH 6 of 6] help: document about "export" template keywords

[Yuya Nishihara]

# HG changeset patch
# User Yuya Nishihara <yuya at tcha.org>
# Date 1538574499 -32400
#      Wed Oct 03 22:48:19 2018 +0900
# Node ID 4b485ae732aea8285de993cecc59e60a22780c53
# Parent  5657857137840bbb4406c7d5ca2b17b99918eb94
help: document about "export" template keywords

diff --git a/mercurial/commands.py b/mercurial/commands.py
--- a/mercurial/commands.py
+++ b/mercurial/commands.py
@@ -2033,6 +2033,14 @@ def export(ui, repo, *changesets, **opts
 
     .. container:: verbose
 
+      Template:
+
+      The following keywords are supported in addition to the common template
+      keywords and functions. See also :hg:`help templates`.
+
+      :diff:    String. Diff content.
+      :parents: List of strings. Parent nodes of the changeset.
+
       Examples:
 
       - use export and import to transplant a bugfix to the current