Refinement of Trace Span Specification Documentation

[liucheng-ms]

Description

This PR introduces enhancements to the Markdown file detailing the trace span specification. Key changes include the addition of punctuation at the end of sentences for improved readability and the elevation of previously bolded text to higher-level titles for better document structure. These updates aim to provide clearer and more accessible documentation for contributors.

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• The pull request does not introduce [breaking changes].
• CHANGELOG is updated for new features, bug fixes or other significant changes.
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