Updated whitespace documentation

[yuvimittal]

updated documentation on difference between @stdlib/regexp/whitespace and \s

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Coverage Report

Package	Statements	Branches	Functions	Lines
regexp/whitespace	$\color{green}496/496$ $\color{green}+100.00\%$	$\color{green}22/22$ $\color{green}+100.00\%$	$\color{green}2/2$ $\color{green}+100.00\%$	$\color{green}496/496$ $\color{green}+100.00\%$

The above coverage report was generated for the changes in this PR.