docs: Update x-envoy-upstream-service-time description to be more accurate

[wbpcode]

Commit Message: Update docs of x-envoy-upstream-service-time.

From the source code, x-envoy-upstream-service-time includes the latency of the network from envoy to upstream host and the time cost of upstream host generating responses. But the description in the docs is ambiguous and may lead to confusion in understanding it. Relevant: #12437.

envoy/source/common/router/router.cc

Lines 1268 to 1276 in d58bdcf

	if (DateUtil::timePointValid(downstream_request_complete_time_)) {
	Event::Dispatcher& dispatcher = callbacks_->dispatcher();
	MonotonicTime response_received_time = dispatcher.timeSource().monotonicTime();
	std::chrono::milliseconds ms = std::chrono::duration_cast<std::chrono::milliseconds>(
	response_received_time - downstream_request_complete_time_);
	if (!config_.suppress_envoy_headers_) {
	headers->setEnvoyUpstreamServiceTime(ms.count());
	}
	}

Risk Level: N/A
Testing: N/A
Docs Changes: Docs only
Release Notes: N/A

Signed-off-by: wbpcode comems@msn.com

[dio]

Thanks!

[dio]

Superfluous spaces? Probably setting your editor to trim empty spaces will be helpful.

[wbpcode]

/retest

[repokitteh-read-only]

🤷‍♀️ nothing to rebuild.

🐱

Caused by: a #12517 (comment) was created by @wbpcode.

see: more, trace.

[wbpcode]

It's ok now.