Updte documentation for Telemetry WithSpan

[yasmin-aumeeruddy]

This doc ought to have some sort of note to explain that internal methods cannot be annoted with @WithSpan

For example, a child span would not be created by doing this but users may expect it to do so:

@GET
@Path("/{id}")
@WithSpan("Parent")
public void parentSan(@PathParam("id") @SpanAttribute("Id") Long id) {
    // Call the child span method
    childSpan(url);
}

@WithSpan("Child Find By Id")
public void childSpan(URL url) {
    // Print the current span ID within the child span
    Span.current().setAttribute("Child Span ID", Span.current().getSpanContext().getSpanId());
    System.out.println("Child Span ID after parent scope: " + Span.current().getSpanContext().getSpanId());
    System.out.println("Child Trace ID after parent scope: " + Span.current().toString());
}

[yasmin-aumeeruddy]

Docs issue: OpenLiberty/docs#7568