Fixed #18731 -- Added an example about customizing "makemessages" command.

Thanks claudp for the suggestion and review.

docs/ref/django-admin.txt

@@ -666,6 +666,11 @@ Use the ``--keep-pot`` option to prevent Django from deleting the temporary
 .pot files it generates before creating the .po file. This is useful for
 debugging errors which may prevent the final language files from being created.
 
+.. seealso::
+
+    See :ref:`customizing-makemessages` for instructions on how to customize
+    the keywords that :djadmin:`makemessages` passes to ``xgettext``.
+
 makemigrations [<app_label>]
 ----------------------------
 

docs/topics/i18n/translation.txt

@@ -1455,6 +1455,42 @@ translation utilities with a ``gettext`` package if the command ``xgettext
 --version`` entered at a Windows command prompt causes a popup window saying
 "xgettext.exe has generated errors and will be closed by Windows".
 
+.. _customizing-makemessages:
+
+Customizing the ``makemessages`` command
+----------------------------------------
+
+.. highlightlang:: python
+
+If you want to pass additional parameters to ``xgettext``, you need to create a
+custom :djadmin:`makemessages` command and override its ``xgettext_options``
+attribute::
+
+    from django.core.management.commands import makemessages
+
+    class Command(makemessages.Command):
+        xgettext_options = makemessages.Command.xgettext_options + ['--keyword=mytrans']
+
+If you need more flexibility, you could also add a new argument to your custom
+:djadmin:`makemessages` command::
+
+    from django.core.management.commands import makemessages
+
+    class Command(makemessages.Command):
+
+        def add_arguments(self, parser):
+            super(Command, self).add_arguments(parser)
+            parser.add_argument('--extra-keyword', dest='xgettext_keywords',
+                                action='append')
+
+        def handle(self, *args, **options):
+            xgettext_keywords = options.pop('xgettext_keywords')
+            if xgettext_keywords:
+                self.xgettext_options = (
+                    makemessages.Command.xgettext_options[:] +
+                    ['--keyword=%s' % kwd for kwd in xgettext_keywords]
+                )
+            super(Command, self).handle(*args, **options)
 
 Miscellaneous
 =============