Checklist

• I've run bundle exec rspec from the root directory to see all new and existing tests pass
• I've followed the fastlane code style and run bundle exec rubocop -a to ensure the code style is valid
• I've read the Contribution Guidelines
• I've updated the documentation if necessary.

Motivation and Context

• is_string is slowly being replaced by type to make the docs clearer, options safer, and Swift generation more correct.
• This PR does this for only appledoc action.

Description

• Remove is_string: false/true from options

Testing Steps

• No functionality changed, all existing unit tests should pass.