[tldoc] Update default value of hash_extra

[Karl Berry]

    > The default value of hash_extra is 50000.

    But this default value is now 600000. 

Thanks, I'll update it.

    Also, I'm not sure if the description
    Only 10,000 control sequences can be stored in the main hash table
    in the same section is still accurate or need an update too.

I'll look into it. Thanks again. -k