templater: add i18n comments to error messages of newly added functions

This patch adds i18n comments to error messages of newly added
functions "startswith" (introduced by 4a445dc5abff) and "word" (by
8f23f8096606).

mercurial/templater.py

@@ -468,6 +468,7 b' def sub(context, mapping, args):'
 
 def startswith(context, mapping, args):
     if len(args) != 2:
+        # i18n: "startswith" is a keyword
         raise error.ParseError(_("startswith expects two arguments"))
 
     patn = stringify(args[0][0](context, mapping, args[0][1]))
@@ -480,6 +481,7 b' def startswith(context, mapping, args):'
 def word(context, mapping, args):
     """return nth word from a string"""
     if not (2 <= len(args) <= 3):
+        # i18n: "word" is a keyword
         raise error.ParseError(_("word expects two or three arguments, got %d")
                                % len(args))