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

In https://github.com/open-telemetry/opentelemetry-ruby/pull/1517#discussion_r1379707498, @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:
https://github.com/open-telemetry/opentelemetry-ruby/blob/a9bcbda1ffca0e781e174da7bdaeb1a83394c595/api/lib/opentelemetry/baggage.rb#L27-L29

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

Start with the linked discussion and the YARD experimental type parser documentation, then inspect the example in api/lib/opentelemetry/baggage.rb at lines 27-29. Search the repository for the [optional Type] pattern and determine the desired replacement; the work is done when the adopted convention supports YARD's parser consistently.