Add info on new function ngCM_Input_Path

src/include/ngspice/sharedspice.h

@@ -45,6 +45,11 @@ The caller may then directly assess the vector data (but probably should
 not modify them).
 
 ***************** If XSPICE is enabled *************************************
+**
+ngCM_Input_Path(const char*)
+sets the input path for files loaded by code models.
+If NULL is sent, return the current Infile_Path.
+
 **
 ngGet_Evt_NodeInfo(char*)
 receives the name of a event node vector (may be in the form 'vectorname' or