Update $urltransform documentation

docs/general/ad-filtering/create-own-filters.md

@@ -2909,18 +2909,35 @@ If multiple `$urltransform` rules match a single request, we will apply each of
 
 **Syntax**
 
-`$urltransform` syntax is similar to replacement with regular expressions [in Perl](http://perldoc.perl.org/perlrequick.html#Search-and-replace).
+`$urltransform` value is a series of one or more transformations separated by `|`. The first transformation is
+applied to the input URL. Each of the following transformations is applied to the output of the previous one.
+The output of a failed transformation (for example, if Base64 decoding failed or if substitution found no matches)
+is its input, unchanged. Formally:
 
 ```text
-urltransform = "/" regexp "/" replacement "/" modifiers
+urltransform = transforms
+transforms = transform | transform "|" transforms
+transform = substitute | decode
+substitute = "/" regexp "/" replacement "/" modifiers
+decode = "b64" | "pct"
 ```
 
-- **`regexp`** — a regular expression.
-- **`replacement`** — a string that will be used to replace the string corresponding to `regexp`.
-- **`modifiers`** — a regular expression flags. For example, `i` — insensitive search, or `s` — single-line mode.
+- **`substitute`** is similar to replacement with regular expressions [in Perl](https://perldoc.perl.org/perlrequick.html#Search-and-replace).
+    - **`regexp`** — a regular expression.
+    - **`replacement`** — a string that replaces whatever is matched by `regexp`. `$1`, `$2`, etc. in the replacement string are replaced with the contents of the corresponding capture group.
+    - **`modifiers`** — regular expression flags, e.g., `i` for case-insensitive search.
+- **`b64`** — decodes a [Base64-encoded](https://datatracker.ietf.org/doc/html/rfc4648) string, both the default and the URL-safe alphabets are supported.
+- **`pct`** — decodes a [percent-encoded](https://datatracker.ietf.org/doc/html/rfc3986#section-2.1) string.
 
 In the `$urltransform` value, two characters must be escaped: the comma `,` and the dollar sign `$`. Use the backslash character `\` for this. For example, an escaped comma looks like this: `\,`.
 
+:::info Compatibility
+
+AdGuard products that use a [CoreLibs] version older than 1.20 only support a single
+`substitute` transformation for the value of the `$urltransform` modifier.
+
+:::
+
 **Changing the origin**
 
 :::info Compatibility