Document :concurrent_downloads in gem environment config docs (follow-up to #9339)

[afurm]

What was the end-user or developer problem that led to this PR?

In #9339, concurrent_downloads was fixed so .gemrc values are honored.
However, the .gemrc key list shown by gem environment still did not mention :concurrent_downloads, which can make users think it is not a supported config key.

Since this text is used for command documentation (including the command reference), this needs a separate docs-focused PR.

What is your fix for the problem, implemented in this PR?

This PR updates Gem::Commands::EnvironmentCommand#description to include:

• :concurrent_downloads: The number of gem downloads to perform concurrently

It also adds a regression test to ensure the description continues to mention this key:

• test_description_mentions_concurrent_downloads in test/rubygems/test_gem_commands_environment_command.rb

I chose this minimal approach because it directly updates the command’s canonical help text and adds test coverage to prevent future drift.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags

[colby-swandale]

Nice one! Much appreciated.