Support YARD's experimental type parser by removing "optional" from type descriptions

[kaylareopelle]

In #1517 (comment), @olleolleolle brought to our attention the YARD experimental type parser does not support [optional Type] argument descriptions in the documentation. One suggested alternative approach is to include the word optional in the description of the argument. This pattern is common in the opentelemetry-ruby repo.

Here's one place where the [optional Type] pattern is used:

opentelemetry-ruby/api/lib/opentelemetry/baggage.rb

Lines 27 to 29 in a9bcbda

	# @param [optional Context] context The context to update with with new
	# modified baggage. Defaults to +Context.current+
	# @return [Context]

Let's revisit our pattern for declaring optional types and determine if we want to adopt a change that supports YARD's type parser.

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