Enhance documentation to show that "ro" is the correct way to say "readonly"

in the mode= query parameter.

FossilOrigin-Name: 788b96851d9ced84757c48dc3e0414cab27ee7e50e9730dab30b2e42a7762397

src/sqlite.h.in

@@ -3499,6 +3499,7 @@ void sqlite3_progress_handler(sqlite3*, int, int(*)(void*), void*);
 **          that uses dot-files in place of posix advisory locking.
 ** <tr><td> file:data.db?mode=readonly <td> 
 **          An error. "readonly" is not a valid option for the "mode" parameter.
+**          Use "ro" instead:  "file:data.db?mode=ro".
 ** </table>
 **
 ** ^URI hexadecimal escape sequences (%HH) are supported within the path and