runtime/pprof: Mention goroutine label heritability

Document goroutine label inheritance. Goroutine labels are copied upon
goroutine creation and there is a test enforcing this, but it was not
mentioned in the docstrings for `Do` or `SetGoroutineLabels`.

Add notes to both of those functions' docstrings so it's clear that one
does not need to set labels as soon as a new goroutine is spawned if
they want to propagate tags.

Updates #32223
Updates #23458

Change-Id: Idfa33031af0104b884b03ca855ac82b98500c8b4
Reviewed-on: https://go-review.googlesource.com/c/go/+/189317


Reviewed-by: Ian Lance Taylor <iant@golang.org>