Stop mentioning haml_io in the reference [ci skip]

Close #1136

CHANGELOG.md

@@ -100,6 +100,8 @@ Released on September 21, 2022
          * Removed: `block_is_haml?`, `capture_haml`, `escape_once`, `find_and_preserve`, `flatten`, `haml_concat`,
            `haml_indent`, `haml_tag`, `haml_tag_if`, `html_attrs`, `html_escape`, `init_haml_helpers`, `is_haml?`,
            `list_of`, `non_haml`, `precede`, `succeed`, `surround`, `tab_down`, `tab_up`, `with_tabs`
+     * `:ruby` filter
+         * Removed: `haml_io`
   * Only the following attributes and `aria`/`data` attributes are considered boolean attributes:
      * `allowfullscreen`, `async`, `autobuffer`, `autofocus`, `autoplay`, `checked`, `controls`, `default`,
        `defer`, `disabled`, `download`, `formnovalidate`, `hidden`, `inert`, `ismap`, `itemscope`, `loop`,

REFERENCE.md

@@ -1168,11 +1168,8 @@ HTML escape code for newlines, to preserve nice-looking output. See also
 
 ### `:ruby` {#ruby-filter}
 
-Parses the filtered text with the normal Ruby interpreter. Creates an `IO`
-object named `haml_io`, anything written to it is output into the Haml document.
-Not available if the {Haml::Options#suppress_eval `:suppress_eval`} option is
-set to true. The Ruby code is evaluated in the same context as the Haml
-template.
+Parses the filtered text with the normal Ruby interpreter.
+The Ruby code is evaluated in the same context as the Haml template.
 
 ### `:sass` {#sass-filter}