chore(doc): doocument tracer.ContextWithSpan panic

[genesor]

What does this PR do?

Improve documentation of the tracer.ContextWithSpan function in charge of adding a Span to a context.Context

Motivation

We spent nearly an hour with @igoragoli debugging a future contrib that was not showing this panic properly.
Having clear documentation over a function that can cause a panic is always nice.

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• 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.
• New code is free of linting errors. You can check this by running ./scripts/lint.sh locally.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.

Unsure? Have a question? Request a review!

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-12-24 17:26:37

Comparing candidate commit 87be0c5 in PR branch ben.db/ctx-with-span with baseline commit e744f8a in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 15 metrics, 0 unstable metrics.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 54.80%. Comparing base (e744f8a) to head (87be0c5).
⚠️ Report is 9 commits behind head on main.

Additional details and impacted files

Files with missing lines	Coverage Δ
ddtrace/tracer/context.go	61.90% <ø> (ø)

... and 21 files with indirect coverage changes

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[kakkoyun]

Since this already panics, shall we add an explicit hard assertion (panic) that could make it easier to explain.

[genesor]

@kakkoyun what kind of assertion are you expecting ? A unit test ?