Add documentation to ExampleGroup#idempotently_define_singleton_method
Add documentation to ExampleGroup#idempotently_define_singleton_method

It's hard to understand how the method works at first sight.

azbshiri/rspec-core,azbshiri/rspec-core,xmik/rspec-core,azbshiri/rspec-core,rspec/rspec-core,rspec/rspec-core,xmik/rspec-core,xmik/rspec-core,rspec/rspec-core