tpl: Add a warnf template func

Fixes #6628

content/en/functions/errorf.md

@@ -1,7 +1,6 @@
 ---
-title: errorf
+title: errorf and warnf
-linktitle: errorf
+description: Log ERROR or WARNING from the templates.
-description: Log ERROR and fail the build from the templates.
 date: 2017-09-30
 publishdate: 2017-09-30
 lastmod: 2017-09-30
@@ -18,12 +17,18 @@ deprecated: false
 aliases: []
 ---
 
-`errorf` will evaluate a format string, then output the result to the ERROR log (and only once per error message to avoid flooding the log).
+`errorf` or `warnf` will evaluate a format string, then output the result to the ERROR or WARNING log (and only once per error message to avoid flooding the log).
 
-This will also cause the build to fail (the `hugo` command will `exit -1`).
+Any ERROR will also cause the build to fail (the `hugo` command will `exit -1`).
+
+Note that the WARNING will only be printed to the console.
 
 ```
 {{ errorf "Failed to handle page %q" .Path }}
 ```
 
-Note that `errorf` supports all the formatting verbs of the [fmt](https://golang.org/pkg/fmt/) package.
+```
+{{ warnf "You should update the shortcodes in %q" .Path }}
+```
+
+Note that `errorf` and `warnf` support all the formatting verbs of the [fmt](https://golang.org/pkg/fmt/) package.

data/docs.json

@@ -3053,6 +3053,22 @@
               "works!\n"
             ]
           ]
+        },
+        "Warnf": {
+          "Description": "Warnf formats according to a format specifier and logs a WARNING.\nIt returns an empty string.",
+          "Args": [
+            "format",
+            "a"
+          ],
+          "Aliases": [
+            "warnf"
+          ],
+          "Examples": [
+            [
+              "{{ warnf \"%s.\" \"warning\" }}",
+              ""
+            ]
+          ]
         }
       },
       "hugo": {