Is the problem with me or with the documentation?

The Sinatra FAQ makes mention of the “escape_html()” method of
Rack::Utils. It links to the Rack::Utils documentation, , but this page
has no mention of escape_html(). Digging into the source code of
lib/rack/utils.rb, however, I am able to find escape_html().

So my question is: why doesn’t the documentation mention this method? Am
I looking in the wrong place? I expect Ruby library documentation to
often be lacking meaningful description (sadly), but this method isn’t
mentioned at all! Isn’t ruby documentation auto-generated from the
source code? Shouldn’t that automatically cause existence of this method
be mentioned in the docs?