Correct description of short-circuit evaluation

Add that final value in a conditional chain may be anything and provide an example
Return NaN for median of vectors containing NaN. Closes #6486
Merge pull request #6819 from simonp0420/patch-1

Correct description of short-circuit evaluation
Merge pull request #6820 from JuliaLang/sjk/median_nan

Return NaN for median of vectors containing NaN. Closes #6486