AIT-8312 tracer/option: document service defaulting

[ahmed-mez]

What does this PR do?

Explain in a code comment why we don't set a global service name if defaulted to binary name.

Motivation

Address some confusing behavior that could be interpreted as bug (see #2240)

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.

For Datadog employees:

• If this PR touches code that handles credentials of any kind, such as Datadog API keys, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

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