[docs] Add priority sampling to ddtrace-run usage

[Kyle-Verhoog]

This fixes the discrepancy between:

dd-trace-py/ddtrace/commands/ddtrace_run.py

Lines 16 to 32 in 57e9954

	Execute the given Python program after configuring it to emit Datadog traces.
	Append command line arguments to your program as usual.
	Usage: [ENV_VARS] ddtrace-run <my_program>
	Available environment variables:
	DATADOG_ENV : override an application's environment (no default)
	DATADOG_TRACE_ENABLED=true|false : override the value of tracer.enabled (default: true)
	DATADOG_TRACE_DEBUG=true|false : override the value of tracer.debug_logging (default: false)
	DATADOG_PATCH_MODULES=module:patch,module:patch... e.g. boto:true,redis:false : override the modules patched for this execution of the program (default: none)
	DATADOG_TRACE_AGENT_HOSTNAME=localhost: override the address of the trace agent host that the default tracer will attempt to submit to (default: localhost)
	DATADOG_TRACE_AGENT_PORT=8126: override the port that the default tracer will submit to (default: 8126)
	DATADOG_SERVICE_NAME : override the service name to be used for this program (no default)
	This value is passed through when setting up middleware for web framework integrations.
	(e.g. pylons, flask, django)
	For tracing without a web integration, prefer setting the service name in code.

and

https://github.com/DataDog/dd-trace-py/blame/master/docs/advanced_usage.rst#L374-L395

We should consider a better way of ensuring these stay synchronized (or if we even need to keep them separated).