Document pattern matching
I noticed 2.7 was released without any official pattern matching documentation, other than the link to the (outdated) presentation.
I don't know, maybe this work is already in progress internally, or maybe it consciously left this way, but I'd like it to be fixed for wider feature adoption.
In this PR, I am trying to document PM to the best of my understanding: https://github.com/ruby/ruby/pull/2786.
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