doc: warn about misuse of strncpy and wcsncpy. * doc/posix-functions/strcpy.texi: Describe requirements on prior memory allocation. * doc/posix-functions/wcscpy.texi: Likewise. * doc/posix-functions/strncpy.texi: Describe what this function is not useful for. * doc/posix-functions/wcsncpy.texi: Likewise.