serial: doc: Un-document obsolete tmpbuf_sem

uart_info.tmpbuf and uart_info.tmpbuf_sem were removed in v2.6.10, in
full-history-linux commit a797ad7e3ae9cad4 ("[SERIAL] Clean up
serial_core.c write functions.").

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>