documentclass[10pt,a4paper]{article}
usepackage[utf8]{inputenc}
usepackage[T1]{fontenc}
usepackage{empheq}
usepackage{bigstrut}

begin{document}

        [ f(x)tobegin{dcases}parbox{40mm}{$f(x)$ is a nice emph{función}, so I want to add this multiline commentbigstrut[b]}
            end{dcases} ]%

        begin{empheq}[left = f(x)toempheqlbrace]{equation*}
          parbox{40mm}{$f(x)$ is a nice emph{función}, so I want to add this multiline comment}
        end{empheq}

end{document}