Add documentation on how to remove the span and disable implicit child spans for a particular code path

Currently, the best way to how to remove the span and disable implicit child spans for a particular code path is probably this

try (Scope ignored = TracingContextUtils.currentContextWith(DefaultSpan.getInvalid())) {
  // Since scoped span is invalid, no tracing happens.
}

This should be documented, possibly with a helper somewhere, something like Scope withTracingDisabled().

/cc @marcingrzejszczak

The issue names no target file or test; start with the Java example using TracingContextUtils and DefaultSpan.getInvalid(), then locate the existing tracing-context documentation. Done means the documented guidance explains how to remove a span and suppress implicit child spans for one code path, and clearly resolves whether a helper such as Scope withTracingDisabled() is needed.