Add short description of the new interface functions

ngSpice_LockRealloc(void) ad ngSpice_UnlockRealloc(void)

src/include/ngspice/sharedspice.h

@@ -82,6 +82,13 @@ char** ngSpice_AllVecs(char*);
 returns to the caller a pointer to an array of vector names in the plot
 named by the string in the argument.
 
+**
+int ngSpice_LockRealloc(void)
+int ngSpice_UnlockRealloc(void)
+Locking and unlocking the realloc of output vectors during simulation. May be set
+during reading output vectors in the primaty thread, while the simulation in the
+background thread is moving on.
+
 **
 Additional basics:
 No memory mallocing and freeing across the interface: