doc: update the SQLite C interoperability example

doc/docs.md

@@ -1713,15 +1713,29 @@ fn main(){
 #flag -lsqlite3
 #include "sqlite3.h"
 
-struct C.sqlite3
-struct C.sqlite3_stmt
+// See also the example from https://www.sqlite.org/quickstart.html
+struct C.sqlite3{}
+struct C.sqlite3_stmt{}
 
-fn C.sqlite3_open(charptr, C.sqlite3)
-fn C.sqlite3_column_int(stmt C.sqlite3_stmt, n int) int
-// Or just define the type of parameter & leave C. prefix
-fn C.sqlite3_prepare_v2(sqlite3, charptr, int, sqlite3_stmt, charptr) int
-fn C.sqlite3_step(sqlite3)
-fn C.sqlite3_finalize(sqlite3_stmt)
+type FnSqlite3Callback fn(voidptr, int, &charptr, &charptr) int
+
+fn C.sqlite3_open(charptr, &&C.sqlite3) int
+fn C.sqlite3_close(&C.sqlite3) int
+fn C.sqlite3_column_int(stmt &C.sqlite3_stmt, n int) int
+// ... you can also just define the type of parameter & leave out the C. prefix
+fn C.sqlite3_prepare_v2(&sqlite3, charptr, int, &&sqlite3_stmt, &charptr) int
+fn C.sqlite3_step(&sqlite3_stmt)
+fn C.sqlite3_finalize(&sqlite3_stmt)
+fn C.sqlite3_exec(db &sqlite3, sql charptr, FnSqlite3Callback, cb_arg voidptr, emsg &charptr) int
+fn C.sqlite3_free(voidptr)
+
+fn my_callback(arg voidptr, howmany int, cvalues &charptr, cnames &charptr) int {
+    for i in 0..howmany {
+	    print('| ${cstring_to_vstring(cnames[i])}: ${cstring_to_vstring(cvalues[i]):20} ')
+	}
+    println('|')
+    return 0
+}
 
 fn main() {
     path := 'users.db'
@@ -1733,7 +1747,16 @@ fn main() {
     C.sqlite3_step(stmt)
     nr_users := C.sqlite3_column_int(stmt, 0)
     C.sqlite3_finalize(stmt)
-    println(nr_users)
+    println('There are $nr_users users in the database.')
+    //
+    error_msg := charptr(0)
+    query_all_users := 'select * from users'
+    rc := C.sqlite3_exec(db, query_all_users.str, my_callback, 7, &error_msg)
+    if rc != C.SQLITE_OK {
+        eprintln( cstring_to_vstring(error_msg) )
+        C.sqlite3_free(error_msg)
+    }
+    C.sqlite3_close(db)
 }
 ```