Inconsistent double slashes in generated Javadoc URLs

[snicoll]

As discovered @ngocnhan-tran1996 in #43512, the generated Javadoc URLs are currently inconsistent. Some have double slashes and some don't. For instance, the section on configuring multiple datasources has two slashes for DataSource but only one for @Bean yet both have a slash at the end in the configuration.

[philwebb]

I think the original PR covered some of the cases, but not all. The Asciidoctor extension tries to deal with this, but for the DataSource case we're the ones concatenating things.