tracing: add package-level documentation for tracing concepts

[irfansharif]

First two commits are from #58509, and can be ignored here.

Release note: None

[cockroach-teamcity]

This change is 

[tbg]

Thank you!

Reviewed 7 of 7 files at r1, 5 of 5 files at r2, 1 of 1 files at r3, 8 of 8 files at r4.
Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @irfansharif and @knz)

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pkg/util/tracing/doc.go, line 53 at r4 (raw file):

//                          way depend on the result of the second (think of a
//                          goroutine that spins off another that outlives it).
//                          Somewhat confusingly, we still refer to the "first"

It's not that confusing when you consider that even with a parent-child relation, you can still have the child outlive the parent (you shouldn't - but there's nothing stopping you). The relation type, as far as I can tell, is really more of a hint than something that materially effects how the span works.

[irfansharif]

bors r+

[craig]

Build failed:

• GitHub CI (Cockroach)

[irfansharif]

bors r+

[craig]

Build failed:

• GitHub CI (Cockroach)

[irfansharif]

#58211, sigh.

bors r+

[craig]

Build failed:

• GitHub CI (Cockroach)

[irfansharif]

bors r+

[craig]

Build failed:

• GitHub CI (Cockroach)

[irfansharif]

bors r+

[craig]

Build failed:

• GitHub CI (Cockroach)

[irfansharif]

This has to be my unluckiest PR to date, now with:

BenchmarkPgCopyConvertRecord
panic: runtime error: slice bounds out of range [1:0]

bors r+

[craig]

Build succeeded:

• GitHub CI (Cockroach)