docs: add about missing protocol-relative link

Author: kenjis

user_guide_src/source/helpers/url_helper.rst

@@ -20,9 +20,9 @@ The following functions are available:
 
 .. php:function:: site_url([$uri = ''[, $protocol = null[, $altConfig = null]]])
 
-    :param  array|string         $uri: URI string or array of URI segments
-    :param  string        $protocol: Protocol, e.g., 'http' or 'https'
-    :param  \\Config\\App $altConfig: Alternate configuration to use
+    :param  array|string         $uri: URI string or array of URI segments.
+    :param  string        $protocol: Protocol, e.g., 'http' or 'https'. If empty string '' is set, a protocol-relative link is returned.
+    :param  \\Config\\App $altConfig: Alternate configuration to use.
     :returns: Site URL
     :rtype:    string
 
@@ -56,8 +56,8 @@ The following functions are available:
 
 .. php:function:: base_url([$uri = ''[, $protocol = null]])
 
-    :param  array|string   $uri: URI string or array of URI segments
-    :param  string  $protocol: Protocol, e.g., 'http' or 'https'
+    :param  array|string   $uri: URI string or array of URI segments.
+    :param  string  $protocol: Protocol, e.g., 'http' or 'https'. If empty string '' is set, a protocol-relative link is returned.
     :returns: Base URL
     :rtype: string
 
@@ -83,6 +83,11 @@ The following functions are available:
     The above example would return something like:
     **http://example.com/blog/post/123**
 
+    If you pass an empty string ``''`` as the second parameter, it returns
+    the protocol-relative link:
+
+    .. literalinclude:: url_helper/026.php
+
     This is useful because unlike :php:func:`site_url()`, you can supply a
     string to a file, such as an image or stylesheet. For example:
 

user_guide_src/source/helpers/url_helper/004.php

@@ -1,3 +1,4 @@
 <?php
 
+// Returns like `http://example.com/blog/post/123`
 echo base_url('blog/post/123');

user_guide_src/source/helpers/url_helper/026.php

@@ -0,0 +1,4 @@
+<?php
+
+// Returns like `//example.com/blog/post/123`
+echo base_url('blog/post/123', '');