docs: update explanation of valid_url_strict

Author: kenjis

user_guide_src/source/libraries/validation.rst

@@ -864,10 +864,10 @@ valid_url               No         Fails if field does not contain (loosely) a
                                    URL. Includes simple strings that could be
                                    hostnames, like "codeigniter".
 valid_url_strict        Yes        Fails if field does not contain a valid URL.  valid_url_strict[https]
-                                   Roughly equivalent to a "fail anything that
-                                   would not be a clickable link." You can
-                                   optionally specify a list of valid schemas.
-                                   If not specified, ``http,https`` are valid.
+                                   You can optionally specify a list of valid
+                                   schemas. If not specified, ``http,https``
+                                   are valid. This rule uses
+                                   PHP's ``FILTER_VALIDATE_URL``.
 valid_date              No         Fails if field does not contain a valid date. valid_date[d/m/Y]
                                    Accepts an optional parameter to matches
                                    a date format.