Update documentation on UpdateInfo::realTime

[azeey]

🦟 Bug fix

Fixes #1777

Summary

The documentation for UpdateInfo::realTime says the value would increase even if simulation is paused. This is definitely not the case as can be seen from the code

gz-sim/src/SimulationRunner.cc

Lines 636 to 639 in 5ad7694

	// Keep track of wall clock time. Only start the realTimeWatch if this
	// runner is not paused.
	if (!this->currentInfo.paused)
	this->realTimeWatch.Start();

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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