[man] Document that FZF_DEFAULT_COMMAND should be POSIX-compliant
[man] Document that FZF_DEFAULT_COMMAND should be POSIX-compliant

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junegunn/fzf,janlazo/fzf,janlazo/fzf,infokiller/fzf,janlazo/fzf,junegunn/fzf,junegunn/fzf,stuha/wbr-zsh,infokiller/fzf,infokiller/fzf