mbstowcs(3) may not return -1

Hello,

While reading the manpage of mbstowcs I noticed an error in the RETURN 
VALUES :

      The mbstowcs() function returns the number of wide characters 
converted,
      not counting any terminating null wide character, or -1 if an invalid
      multibyte character was encountered.

Since size_t is unsigned, it can't returns -1.

Cheers,

-- 
David Demelier