(doc) Remove outdated notice concerning rdoc and ruby > 1.8.7

This note is deprecated by the following pull requests:

https://github.com/puppetlabs/puppet/pull/2009
https://github.com/puppetlabs/puppet/pull/2187

Relevant redmine ticket:

https://projects.puppetlabs.com/issues/22180
Merge pull request #2949 from roidelapluie/Remove_Puppet_Doc_Oudated_Documentation

(doc) Remove outdated notice concerning rdoc and ruby > 1.8.7