This function will modify the external Id (XID) of the specified
Document. The external Id is the (absolute) path of the initial file
that contained the text which was used in order to create the specified
Document. The value is expected to be encoded using the UTF-8 encoding
(thus enabling the existance of non Latin characters in the value). The
caller can use the function tip_ExternalToUft in order to convert a
string from any supported encoding to UTF.
Return Value:
If the external Id of the specified Document is successfully updated,
TCL_OK will be returned. In case of an error, the return value will be
TCL_ERROR and an error message describing the error will be left at the
current Tcl interpreter (CDM_Interp).