tool/var: explain how the null termination byte is there

Closes #19287

src/var.c

@@ -362,6 +362,7 @@ static ParameterError addvariable(const char *name,
   p = calloc(1, sizeof(struct tool_var) + nlen);
   if(p) {
     memcpy(p->name, name, nlen);
+    /* the null termination byte is already present from above */
 
     p->content = contalloc ? content : memdup0(content, clen);
     if(p->content) {