Add a yieldself tag

[castwide]

Description

Add a @yieldself tag to document the context in which yielded blocks are expected to run. Example:

class Foo; end

class Bar
  def initialize
    @foo = Foo.new
  end

  # @yieldself [Foo]
  def run_in_foo &proc
    @foo.instance_eval &proc
  end
end

Solargraph currently uses this tag to enhance autocompletion. I was planning to write an extension so gems could add it to their yardocs, but it seemed useful enough in documentation to check if it was worth adding to YARD itself.

Completed Tasks

• I have read the Contributing Guide.
• The pull request is complete (implemented / written).
• Git commits have been cleaned up (squash WIP / revert commits).
• I wrote tests and ran bundle exec rake locally (if code is attached to PR).