diff --git a/ext/stringio/stringio.c b/ext/stringio/stringio.c
index aaf008ad..7b2ec144 100644
--- a/ext/stringio/stringio.c
+++ b/ext/stringio/stringio.c
@@ -225,17 +225,32 @@ strio_s_allocate(VALUE klass)
  * call-seq:
  *   StringIO.new(string = '', mode = 'r+') -> new_stringio
  *
- * Note that +mode+ defaults to <tt>'r'</tt> if +string+ is frozen.
- *
  * Returns a new \StringIO instance formed from +string+ and +mode+;
- * see {Access Modes}[rdoc-ref:File@Access+Modes]:
+ * the instance should be closed when no longer needed:
+ *
+ *   strio = StringIO.new
+ *   strio.string        # => ""
+ *   strio.closed_read?  # => false
+ *   strio.closed_write? # => false
+ *   strio.close
+ *
+ * If +string+ is frozen, the default +mode+ is <tt>'r'</tt>:
  *
- *   strio = StringIO.new # => #<StringIO>
+ *   strio = StringIO.new('foo'.freeze)
+ *   strio.string        # => "foo"
+ *   strio.closed_read?  # => false
+ *   strio.closed_write? # => true
  *   strio.close
  *
- * The instance should be closed when no longer needed.
+ * Argument +mode+ must be a valid
+ * {Access Mode}[https://docs.ruby-lang.org/en/master/File.html#class-File-label-Access+Modes],
+ * which may be a string or an integer constant:
+ *
+ *   StringIO.new('foo', 'w+')
+ *   StringIO.new('foo', File::RDONLY)
  *
- * Related: StringIO.open (accepts block; closes automatically).
+ * Related: StringIO.open
+ * (passes the \StringIO object to the block; closes the object automatically on block exit).
  */
 static VALUE
 strio_initialize(int argc, VALUE *argv, VALUE self)