Document binding.irb on Binding [ci skip]

For some reason this very useful method was undocumented since it was added in
493e48897421d176a8faf0f0820323d79ecdf94a which makes finding it in the docs
impossible before this change.

I've added a detailed example with sample code because it's one of the most
powerful tools to debug Ruby code and I believe very few people are aware of it
due to the lack of documentation.

[Fix GH-2010]

From: Olivier Lacan <hi@olivierlacan.com>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65674 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

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