Update documentation of `TermApp::Terminal`

- `lines`, `columns`
- `current_user`, `current_board`
- `#clrtoeol`
- `#mvaddstr`
- `#getyx`
- `#mvgetnstr`
- `#erase_body`
- `#erase_all`
- `#getmaxyx`
- `#print_block`
- `#print_header`
- `#print_footer`